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Page No 131:

Question 1:

Name one metal and one non-metal which exist in liquid state at room temperature.

Answer:

Mercury is the metal that exists in liquid state at room temperature. The non-metal that exists in liquid state at room temperature is bromine.

Page No 131:

Question 2:

Why are metals called electropositive elements whereas non-metals are called electronegative elements?

Answer:

Metals can form positive ions easily by losing electrons. So, they are called electropositive elements. Non-metals can form negative ions easily by gaining electrons and so, they are called electronegative elements.

Page No 131:

Question 3:

(a) Name the most abundant metal in the earth's crust.
(b) Name the most abundant non-metal in the earth's crust.

Answer:

(a) Aluminium is the most abundant metal in the earth's crust.
(b) Silicon is the most abundant non-metal in the earth's crust.



Page No 132:

Question 4:

Name one metal which has a low melting point.

Answer:

Sodium is a metal that has a low melting point.

Page No 132:

Question 5:

Name the metal which is the poorest conductor of heat.

Answer:

Lead is the metal that is the poorest conductor of heat.

Page No 132:

Question 6:

State whether the following statement is true or false:
Non-metals react with dilute acids to produce a gas which burns with a 'pop' sound.

Answer:

False. Metals react with dilute acids to produce hydrogen gas that burns with a 'pop' sound.

Page No 132:

Question 7:

From amongst the metals sodium, calcium, aluminium, copper, and magnesium, name the metal:
(i) which reacts with water only on boiling, and
(ii) another which does not react even with steam.

Answer:

(i) Magnesium reacts with water only on boiling.
(ii) Copper does not react even with steam.

Page No 132:

Question 8:

What changes in the colour of iron nails and copper sulphate solution do you observe after keeping the iron nails dipped in copper sulphate solution for about 30 minutes?

Answer:

An iron nail displaces copper from a copper sulphate solution to form a ferrous sulphate solution, due to which the blue colour of the copper sulphate solution fades. Also, copper is deposited on the iron nail, which turns it reddish-brown in colour.

Page No 132:

Question 9:

What is aqua-regia? Name two special metals which are insoluble in common reagents but dissolve in aqua-regia.

Answer:

Aqua-regia is a freshly prepared mixture of one part of concentrated nitric acid and three parts of concentrated hydrochloric acid. Gold and platinum are the special metals that are insoluble in common reagents but dissolve in aqua-regia.

Page No 132:

Question 10:

Give the names and formulae of (a) two acidic oxides, and (b) two basic oxides.

Answer:

(a) Carbon dioxide with the formula CO2 and sulphur dioxide with the formula SO2 are two acidic oxides.
(b) Sodium oxide with the formula Na2O and potassium oxide with the formula K2O are two basic oxides.

Page No 132:

Question 11:

What name is given to those metal oxides which show basic as well as acidic behaviour?

Answer:

Metal oxides that show basic as well as acidic behaviour are called amphoteric oxides. Aluminium oxide is an example.

Page No 132:

Question 12:

Name two metals which form amphoteric oxides.

Answer:

Aluminium and zinc are two metals that form amphoteric oxides.

Page No 132:

Question 13:

A copper coin is kept immersed in a solution of silver nitrate for some time. What will happen to the coin and the colour of the solution?

Answer:

When a copper coin is kept immersed in a solution of silver nitrate for some time, the colour of the coin becomes shiny greyish-white due to the deposition of silver on it. The solution turns blue due to the formation of copper nitrate.

Page No 132:

Question 14:

Which property of copper and aluminium makes them suitable:
(a) for making cooking utensils and boilers?
(b) for making electric wires?

Answer:

(a) Copper and aluminium are good conductors of heat. This property makes them suitable for making cooking utensils and boilers.
(b) Copper and aluminium are good conductors of electricity. This property makes them suitable for making electric wires.

Page No 132:

Question 15:

Write the names and formulae of (a) a metal hydride, and (b) a non-metal hydride.

Answer:

(a) Sodium hydride is a metal hydride with the formula NaH.
(b) Hydrogen sulphide is a non-metal hydride with the formula H2S.

Page No 132:

Question 16:

Name the metal which has been placed:
(a) at the bottom of the reactivity series
(b) at the top of the reactivity series
(c) just below copper in the reactivity series

Answer:

(a) Gold is the metal that has been placed at the bottom of the reactivity series.
(b) Potassium is the metal that has been placed at the top of the reactivity series.
(c) Mercury is the metal that has been placed just below copper in the reactivity series.

Page No 132:

Question 17:

Which of the two metals is more reactive: copper or silver?

Answer:

Of the two metals, copper is more reactive than silver.

Page No 132:

Question 18:

(a) Name one metal which is stored in kerosene oil.
(b) Name one non-metal which is stored under water.

Answer:

(a) Sodium is a metal that is stored in kerosene oil.
(b) White phosphorus is a non-metal that is stored under water.

Page No 132:

Question 19:

Write equation for the reaction of:
(a) sodium with oxygen
(b) magnesium with oxygen

Answer:

(a)  4Na (s) +  O2(g)              2Na2O (s)
    Sodium       Oxygen          Sodium oxide

(b)  2Mg (s)   +    O2(g)                2MgO (s) 
    Magnesium     Oxygen           Magnesium oxide

Page No 132:

Question 20:

Name two metals which are used:
(a) for making electric wires:
(b) for making domestic utensils and factory equipment.
(c) for making jewellery and to decorate sweets.

Answer:

(a) Copper and aluminium are two metals used to make electric wires.
(b) Iron and aluminium are two metals used to make domestic utensils and factory equipment.
(c) Gold and silver are two metals used to make jewellery and to decorate sweets.

Page No 132:

Question 21:

Which metal foil is used for packing some of the medicine tablets?

Answer:

Aluminium metal foil is used to pack some medicine tablets.

Page No 132:

Question 22:

Name the non-metal which is used:
(a) to convert vegetable oil into vegetable ghee (solid fat).
(b) as a rocket fuel (in liquid form).
(c) to make electrodes of dry cells.
(d) to preserve food materials.
(e) in the vulcanisation  of rubber.

Answer:

(a) Hydrogen is the non-metal that is used to convert vegetable oil into vegetable ghee (solid fat).
(b) Hydrogen is the non-metal that is used as a rocket fuel (in liquid form).
(c) Carbon is the non-metal that is used to make electrodes of dry cells.
(d) Nitrogen is the non-metal that is used to preserve food materials.
(e) Sulphur is the non-metal that is used in the vulcanisation of rubber.

Page No 132:

Question 23:

Name one property which is characteristic of (a) metals, and (b) non-metals.

Answer:

(a) Metals are malleable and ductile. That is, metals can be hammered into thin sheets and drawn into thin wires.
(b) Non-metals are brittle, i.e. they break easily.

Page No 132:

Question 24:

What is meant by "brittleness"? Which type of elements usually show brittleness: metals or non-metals?

Answer:

Brittleness is the property of being broken easily. Non-metals are the elements that usually show brittleness.

Page No 132:

Question 25:

What will happen if a strip of zinc is immersed in a solution of copper sulphate?

Answer:

If a zinc strip is immersed in a solution of copper sulphate, the blue colour of copper sulphate fades due to the formation of colourless zinc sulphate. The metal, copper, reddish-brown in colour is deposited on the zinc strip.
CuSO4(aq) +  Zn(s) ZnSO4(aq) + Cu(s)

Page No 132:

Question 26:

What will happen if a strip of copper is kept immersed in a solution of silver nitrate (AgNO3)?

Answer:

If a strip of copper is kept immersed in a solution of silver nitrate, the solution gradually becomes blue due to the formation of copper nitrate and a shiny greyish-white deposit of silver is formed on the copper strip.
2AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag(s)

Page No 132:

Question 27:

What happens when iron nails are put into copper sulphate solution?

Answer:

When iron nails are put into copper sulphate solution, the blue colour of copper sulphate fades gradually to green due to the formation of ferrous sulphate and the reddish-brown copper metal is deposited on the iron nails.
CuSO4(aq) + Fe(s) FeSO4(aq) + Cu(s)  

Page No 132:

Question 28:

How would you show that silver is chemically less reactive than copper?

Answer:

When silver is dipped in a copper sulphate solution, no reaction occurs. This means silver cannot displace copper from the copper sulphate solution. Hence, it shows that silver is chemically less reactive than copper.

Page No 132:

Question 29:

Give reason for the following:
Blue colour of copper sulphate solution is destroyed when iron filings are added to it.

Answer:

When iron fillings are added to copper sulphate solution, iron will displace copper to form iron sulphate. The blue colour of the copper sulphate solution changes to green as iron sulphate is green in colour.



Page No 133:

Question 36:

(a) What is meant by saying that the metals are malleable and ductile? Explain with examples.
(b) Name two metals which are both malleable and ductile.
(c) Which property of iron metal is utilised in producing iron sheets required fro making buckets?
(d) Which property of copper metal is utilised in making thin wires?

Answer:

(a) Metals are malleable, which means that they can be hammered to form thin sheets. For example, gold and silver.
Metals are ductile, which means that they can be stretched to form thin wires. For example, copper and aluminium.

(b) Gold and copper are two metals that are both malleable and ductile.

(c) Malleability is the property of iron that is utilised to produce iron sheets required for to make buckets.

(d) Ductility is the property of copper that is utilised to make thin wires.

Page No 133:

Question 37:

Name two metals which react violently with cold eater. Write any three observations you would make when such a metal is dropped into water.

Answer:

Sodium and potassium are two metals that react violently with cold water.

Three observations when such a metal is dropped in water are:

  1. Reaction is highly exothermic i.e. a huge amount of heat is produced.
  2. Hydrogen gas is evolved in the reaction.
  3. A small explosion takes place because the heat produced in the reaction burns the hydrogen gas. Hydrogen can be identified by the 'pop' sound it makes while burning.

Page No 133:

Question 38:

(a) With the help of examples, describe how metal oxides differ from non-metal oxides.
(b) Which of the following elements would yield: (i) an acidic oxide, (ii) a basic oxide, and (iii) a neutral oxide?
Na, S, C, K, H

Answer:

(a) Metal oxides are generally basic in nature, which can turn red litmus solution to blue while non-metal oxides are generally acidic in nature, which can turn blue litmus solution to red. For example, sodium oxide and potassium oxide are metal oxides which are basic in nature; carbon dioxide and sulphur dioxide are non-metal oxides which are acidic in nature.

(b) (i) S and C would yield acidic oxides.
      (ii) Na and K would yield basic oxides.
      (iii) H would yield neutral oxides.

Page No 133:

Question 39:

(a) What are amphoteric oxides? Give two examples of amphoteric oxides.
(b) Choose the acidic oxides, basic oxides and neutral oxides from the following:
Na2O; CO2; CO; SO2; MgO; N2O; H2O.
(c) Which of the following are amphoteric oxides:
MgO, ZnO, P2O3, Al2O3, NO2

Answer:

(a) Amphoteric oxides are those metal oxides which show basic as well as acidic behaviour. They react with acids as well as bases to form salts and water. Aluminium oxide and zinc oxide are examples.

(b) Acidic oxides: CO2and SO2
Basic oxides: Na2O and MgO
Neutral oxides: N2O, CO and H2O

(c) ZnO and Al2O3 are the amphoteric oxides.

Page No 133:

Question 40:

(a) What is the nature of the oxide SO2? What happens when it is dissolved in water? Write the chemical equation of the reaction involved.
(b) What is the nature of the oxide Na2O? What happens when it is dissolved in water? Write the chemical equation of the reaction involved.

Answer:

(a) SO2 is acidic in nature. When it is dissolved in water, sulphurous acid is formed.
     Chemical equation: SO2(g) + H2O(l) H2SO3(aq)

(b) Na2O is basic in nature. When it is dissolved in water, sodium hydroxide is formed.
     Chemical equation: Na2O(s) + H2O(l) 2NaOH(aq)

Page No 133:

Question 41:

(a) What type of oxides are formed when non-metals react with oxygen? Explain with an example.
(b) What type of oxides are formed when metals combine with oxygen? Explain with the help of an example.

Answer:

(a) When non-metals react with oxygen, acidic oxides are formed. For example, sulphur on reacting with oxygen form sulphur dioxide. This oxide turns blue litmus solution to red, which shows that it is an acidic oxide.

(b) When metals react with oxygen, basic oxides are formed. For example, sodium on reacting with oxygen form sodium oxide. This oxide turns red litmus solution to blue, which shows that it is a basic oxide.

Page No 133:

Question 42:

(a) Explain why, metals usually do not liberate hydrogen gas with dilute nitric acid.
(b) Name two metals which can, however, liberate hydrogen gas from very dilute nitric acid.

Answer:

(a) Nitric acid is a powerful oxidising agent. So as soon as hydrogen gas is produced due to the action of nitric acid on metals, it oxidises hydrogen to form water and reduces itself to nitrogen oxides. Hence hydrogen gas is not liberated in the reaction.

(b) Magnesium and manganese are two metals that liberate hydrogen gas from very dilute nitric acid.

Page No 133:

Question 43:

(a) How do metals react with hydrogen? Explain with an example.
(b) How do non-metals react with hydrogen? Explain with an example.

Answer:

(a) Metals generally do not react with hydrogen. But some reactive metals like sodium, potassium, calcium and magnesium can force the hydrogen atom to accept the electrons given by them and form salt-like ionic solid compounds called metal hydrides.

For example:  
2Na(s) + H2(g) 2NaH(s)
                      
(b) Non-metals react with hydrogen by sharing their electrons with hydrogen electrons to form covalent hydrides.
For example: Sulphur shares its electrons with hydrogen electrons to form hydrogen sulphide that is covalent in nature.
    H2(g) + S(l) H2S (g)
   

Page No 133:

Question 44:

(a) What happens when calcium reacts with chlorine? Write an equation for the reaction which takes place.
(b) What happens when magnesium reacts with very dilute nitric acid? Write an equation for the reaction involved.

Answer:

(a) Calcium reacts vigorously with chlorine to form calcium chloride, which is an ionic chloride.
     Ca(s) + Cl2(g) CaCl2(s)
        
(b) When magnesium reacts with very dilute nitric acid, it forms magnesium nitrate and hydrogen gas is evolved.
As the nitric acid is very dilute, it acts as a weak oxidising agent, which is unable to oxidise the hydrogen gas.
  Mg(s) + 2HNO3(aq) Mg(NO3)2(aq) + H2(g)

Page No 133:

Question 45:

(a) Arrange the following metals in order of their chemical reactivity, placing the most reactive metal first:
Magnesium, Copper, Iron, Sodium, Zinc, Lead, Calcium.
(b) What happens when a rod of zinc metal is dipped into a solution of copper sulphate? Give chemical equation of the reaction involved.

Answer:

(a) Sodium > Calcium > Magnesium > Zinc > Iron > Lead > Copper

(b) When a zinc rod is dipped in a solution of copper sulphate, zinc, being a more reactive metal than copper, will displace copper from copper sulphate solution. The blue colour of the copper sulphate solution gradually fades due to the formation of the green-coloured zinc sulphate. The red-brown colour of copper will be deposited on the zinc rod.
  Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)

Page No 133:

Question 30:

Name a non-metal having a very high melting point.

Answer:

Diamond is a non-metal that has a very high melting point.

Page No 133:

Question 31:

Which property of graphite is utilised in making electrodes?

Answer:

Graphite is a good conductor of electricity and that is why it is used to make electrodes.

Page No 133:

Question 32:

Name two non-metals which are both brittle and non-ductile.

Answer:

Sulphur and phosphorus are two non-metals that are both brittle and non-ductile.

Page No 133:

Question 33:

Explain why, the surface of some metals acquires a dull appearance when exposed to air for a long time.

Answer:

Some metals, when exposed to air for a long time, react with oxygen in the atmosphere to form a dull layer of metal oxide on their surfaces. This makes the metals look dull.

Page No 133:

Question 34:

Complete and balance the following equations:
(a) Na    +    O2                 
(b) Na2O     +     H2O                
(c) Fe (s)      +     H2O (g)  Red heat
(d) Cu(NO3)2 (aq)     +   Zn (s)                    

Answer:

(a)  4Na(s) + O2(g) 2Na2O(s)    

(b)
Na2O(s) + H2O(l) 2NaOH (aq)

(c) 3Fe(s) + 4H2O(g)  Red Heat  Fe3O4(s) + 4H2(g)

(d) Cu(NO3)2(aq)
+ Zn(s) Zn(NO3)2(aq) + Cu(s) (a)(a   Zn (s)                    

Page No 133:

Question 35:

Fill in the following blanks with suitable words:
(a) Magnesium liberates ............... gas on reacting with hot boiling water.
(b) The white powder formed when magnesium ribbon burns in oxygen is of ....................
(c) Ordinary aluminium strips are not attacked by water because of the presence of a layer of ................. on the surface of aluminium.
(d) A metal having low melting point is ..................... but a non-metal having very high melting point is ..............
(e) Calcium is a ................. reactive metal than sodium.

Answer:

(a) Magnesium liberates hydrogen gas on reacting with hot boiling water.
(b) The white powder formed when a magnesium ribbon burns in oxygen is magnesium oxide.
(c) Ordinary aluminium strips are not attacked by water because of the presence of a layer of aluminium oxide on the surface of aluminium.
(d) A metal having low melting point is sodium but a non-metal having very high melting point is diamond.
(e) Calcium is a less reactive metal than sodium.



Page No 134:

Question 46:

A copper plate was dipped in AgNO3 solution. After certain time, silver from the solution was deposited on the copper plate. State the reason why it happened. Give the chemical equation of the reaction involved.

Answer:

When the copper plate is dipped in AgNO3 solution, silver from the solution gets deposited on the copper plate. This happens because copper is a more reactive metal than silver and it displaces silver from the silver nitrate solution.

 2AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag(s)
  

Page No 134:

Question 47:

State five uses of metals and five of non-metals.

Answer:

Five uses of metals are:

  1. Metals are ductile and can be drawn into wires. Example: copper and aluminium
  2. House-hold utensils and factory equipment can be made using metals because they are malleable. Example:Iron, copper and aluminium.
  3. Iron can be protected from rusting by galvanising it with zinc.
  4. Mercury, a liquid metal, is used in thermometers.
  5. Car batteries are made out of the metal, lead.
Five uses of non-metals are:
  1. Hydrogen is used to manufacture ammonia.
  2. Hydrogen is also used in rocket fuel.
  3. Graphite is used to make electrodes of electrolytic cells and dry cells.
  4. Nitrogen, due to its inertness, is used to preserve food.
  5. Sulphur is used to vulcanise rubber.

Page No 134:

Question 48:

State one use each of the following metals:
Copper, Aluminium, Iron, Silver, Gold, Mercury

Answer:

Copper is used to make electrical wires.

Aluminium is used to make house-hold utensils and factory equipment.

In the preparation of ammonia gas by Haber's process, iron acts as a catalyst.

Thin silver foils are used to decorate sweets.

Gold is used to make jewellery.

Liquid mercury is used in thermometers.

Page No 134:

Question 49:

(a) State one use each of the following non-metals:
Hydrogen, Carbon (as Graphite), Nitrogen Sulphur
(b) Name the metal which is used in making thermometers.

Answer:

(a) Hydrogen is used as fuel in rockets.

Carbon (as graphite) is used to make electrodes of electrolytic cells and dry cells.

Nitrogen, due to its inertness, is used to preserve food material.

Sulphur is used as a fungicide.

(b) Liquid mercury is used in thermometers.

Page No 134:

Question 50:

(a) Why does aluminium not react with water under ordinary conditions?
(b) Name two metals which can displace hydrogen from dilute acids.
(c) Name two metals which cannot displace hydrogen from dilute acids.

Answer:

(a) Aluminium does not react with water under ordinary conditions because of the presence of a thin but tough layer of aluminium oxide on its surface, This layer of aluminium oxide prevents it from reacting with water.

(b) Sodium and magnesium are two metals which can displace hydrogen from dilute acids.

(c) Copper and silver are two metals which cannot displace hydrogen from dilute acids.

Page No 134:

Question 51:

(a) Why is sodium kept immersed in kerosene oil?
(b) Why is white phosphorus kept immersed under water?
(c) Can we keep sodium immersed under eater? Why?

Answer:

(a) Sodium is kept immersed in kerosene oil because sodium is a very reactive metal that reacts vigorously with the oxygen in the air and with water. If sodium reacts with oxygen or water, it catches fire in an exothermic reaction.

(b) White phosphorus is kept immersed in water because it starts burning spontaneously when it comes into contact with air. But it does not react with water.

(c) No, we cannot keep sodium immersed in water because it reacts vigorously even with cold water to form sodium hydroxide, hydrogen gas and a huge amount of heat.

 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) + HEAT

Page No 134:

Question 52:

(a) Describe the reaction of potassium with water. Write the equation of the reaction involved.
(b) Write an equation of the reaction of iron with steam. Indicate the physical states of all the reactants and products.
(c) Which gas is produced when dilute hydrochloric acid is added to a reactive metal?

Answer:

(a) Potassium reacts violently with water to form potassium hydroxide and hydrogen gas. The huge amount of heat evolved in the reaction will immediately burn the hydrogen gas produced in the reaction.
 
    2K (s) + 2H2O 2KOH (aq) + H(g) +  HEAT

(b) Red hot iron reacts with steam to form iron (II, III) oxide and hydrogen.
  3Fe (s) + 4H2O Fe3O(s) + 4H(g)
 
(c) When dilute hydrochloric acid is added to a reactive metal, hydrogen gas is produced.

Page No 134:

Question 53:

(a) Give one example, with equation, of the displacement of hydrogen by a metal from an acid.
(b) Name two metals (other than zinc and iron) which can displace hydrogen from dilute hydrochloric acid?

Answer:

(a) Sodium displaces hydrogen from dilute hydrochloric acid.
      2Na(s) + 2HCl(aq) 2NaCl(aq) + H2(g)

(b) Magnesium and aluminium are two metals that can displace hydrogen from dilute hydrochloric acid.

Page No 134:

Question 54:

What is the action of water on (a) sodium (b) magnesium, and (c) aluminium? Write equations of the chemical reactions involved.

Answer:

(a) Sodium reacts with water vigorously to form sodium hydroxide and hydrogen gas with a large amount of heat.
  2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) + HEAT

(b) Magnesium does not react with cold water.
 (i) However, it reacts with steam rapidly to form magnesium hydroxide and hydrogen gas.
       Mg(s) + H2O(g)   Mg(OH)2(aq)  + H2(g)

(ii) Magnesium reacts very rapidly with steam to form magnesium oxide and hydrogen gas.
  Mg(s) + H2O(g)   MgO(s)  +  H2(g)

(c) Aluminium does not react with cold water. But it reacts with steam to form aluminium oxide and hydrogen gas.
   2Al(s) + 3H2O(g) Al2O3(s) +  3H2(g)

Page No 134:

Question 55:

You are given samples of three metals − sodium, magnesium and copper. Suggest any two activities to arrange them in order of their decreasing reactivities.

Answer:

Sodium, magnesium and copper can be arranged based on their reactivity with water. Sodium reacts violently with cold water because it is the most reactive of the three. Magnesium does not react with hot or cold water; it reacts with only steam. Copper does not react even with steam and it is the least reactive of the three.
These three metals can be arranged based on their reactivity with a dilute acid as well. Sodium reacts with dilute hydrochloric acid violently. Magnesium reacts with dilute hydrochloric acid but less rapidly than sodium. Copper is the least reactive and it does not react with dilute hydrochloric acid.
So the order of their decreasing reactivity is sodium > magnesium > copper.

Page No 134:

Question 56:

(a) Write one reaction in which aluminium oxide behaves as a basic oxide and another in which it behaves as an acidic oxide.
(b) What special name is given to substances like aluminium oxide.
(c) Name another metal oxide which behaves like aluminium oxide.

Answer:

(a) When aluminium oxide reacts with hydrochloric acid, it behaves as a basic oxide:
  Al2O3(s) + 6HCl(aq) 2AlCl3(aq) + 3H2O(l)

When aluminium oxide reacts with a base like sodium hydroxide, it behaves as an acidic oxide:
  Al2O3(s) + 2NaOH(aq) 2NaAlO2(aq) +  H2O(l)

(b) Aluminium oxide reacts with both acids and bases and so it is called an amphoteric oxide.

(c) Zinc oxide is another metal oxide that behaves like aluminium oxide.

Page No 134:

Question 57:

(a) What happens when calcium reacts with water? Write the chemical equation of the reaction of calcium with water.
(b) Write the chemical equation of the reaction which takes place when iron reacts with dilute sulphuric acid. What happens when the gas produced is ignited with a burning matchstick?

Answer:

(a) When calcium reacts with water, it forms calcium hydroxide and hydrogen gas. Due to the formation of hydrogen bubbles that stick to the surface of calcium, it starts floating on water.
       Ca(s) + 2H2O(l) Ca(OH)2(aq) +  H2(g)
    
(b) The chemical equation of the reaction between iron and dilute sulphuric acid is as follows:       
        Fe(s) + H2SO4(aq) FeSO4(aq) + H2(g)
        
When the hydrogen gas produced is ignited with a burning matchstick, the gas burns with a 'pop' sound and a small explosion takes place.

Page No 134:

Question 58:

You are given a dry cell, a torch bulb with holder, wires and crocodile clips. How would you use them to distinguish between samples of metals and non-metals?

Answer:

Take a torch bulb fitted with a holder and connect it to a dry cell using connecting wires, as shown in the figure below. There is a gap between the ends of the crocodile clips A and B; so, no current flows in the incomplete circuit. Hence, the bulb does not light up.

Metals are good conductors of electricity. Hence, if samples of metals are inserted between the clips A and B, the sample will allow an electric current to pass through it and the bulb will light up. But non-metals do not conduct electricity. Hence, the samples of non-metals do not allow any electric current to pass through them and the bulb will not glow.

Page No 134:

Question 59:

State any five physical properties of metals and five physical properties of non-metals.

Answer:

Five physical properties of metals are:

  1. Metals are malleable and ductile.
  2. Metals are good conductors of heat and electricity.
  3. Metals are lustrous (shiny) and can be polished.
  4. Metals are solids at room temperature (except mercury, which is liquid).
  5. Metals are tough and strong.
Five physical properties of non-metals are:
  1. Non-metals are brittle. They are neither malleable nor ductile.
  2. Non-metals are bad conductors of heat and electricity (except graphite).
  3. Non-metals are non lustrous (dull) and cannot be polished (except iodine).
  4. Non-metals may be solids, liquids or gases at room temperature.
  5. Non-metals are neither tough nor strong.

Page No 134:

Question 60:

(a) Name two physical properties each of sodium and carbon in which their behaviour is not as expected from their classification as metal and non-metal respectively.
(b) Name two metals whose melting points are so low that they melt when held in the hand.

Answer:

(a) Metals are generally strong. But sodium is not a strong metal and it cannot support heavy objects without breaking.
      Metals are generally hard. But sodium is a soft metal and can be cut even with a knife.
     Non-metals are generally soft. But carbon in the form of diamond is extremely hard.
     Non-metals are generally non-lustrous. But carbon in the form of diamond is a lustrous (shiny) substance.

(b)  Sodium and cesium are the two metals whose melting points are so low that they melt when held in hand.

Page No 134:

Question 61:

Metals are said to be shiny. Why do metals generally appear to be dull? How can their brightness be restored?

Answer:

Metals, when exposed to air for a long time, become dull due to the formation of a thin layer of oxides, carbonates or sulphides on their surfaces. This is because they react with various gases present in the atmosphere. This is why metals corrode. But their brightness can be restored by rubbing them with a sand paper. Then the outer corroded layer of the metals is removed and they become shiny again.

Page No 134:

Question 62:

(a) What are metals? Name five metals.
(b) Name a metal which is so soft that it can be cut with a knife.
(c) Name the metal which is the best conductor of heat and electricity.
(d) What happens when a metal reacts with dilute hydrochloric acid? Explain with the help of an example.
(e) Write the equations for the reaction of:
(i) Magnesium with dilute hydrochloric acid
(ii) Aluminium with dilute hydrochloric acid
(iii) Zinc with dilute hydrochloric acid
(iv) Iron with dilute hydrochloric acid
Name the products formed in each case. Also indicate the physical states of all the substances involved.

Answer:

(a) Metals are generally solid materials that conduct heat and electricity. They are malleable and ductile and also lustrous, hard, strong, heavy and sonorous. Iron, aluminium, copper, gold and silver are five metals.

(b) Sodium is a metal that is so soft that it can be cut with a knife.

(c) Silver is the metal which is the best conductor of heat and electricity.

(d) When a metal reacts with dilute hydrochloric acid, it forms a metal chloride salt and hydrogen gas.
     For example, sodium reacts with dilute hydrochloric acid to form sodium chloride salt and hydrogen gas.
     2Na(s) + 2HCl(aq) 2NaCl(aq) + H2(g)

(e) (i) Magnesium reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen gas.
     Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

(ii) Aluminium reacts with dilute hydrochloric acid to form aluminium chloride and hydrogen gas.
      2Al(s) + 6HCl(aq) 2AlCl3(aq) + 3H2(g)
        
(iii)  Zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas.
        Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

(iv) Iron reacts with dilute hydrochloric acid to form iron (II) chloride and hydrogen gas.
      Fe(s) + 2HCl(aq) FeCl2(aq) + H2(g)
              



Page No 135:

Question 63:

(a) Define non-metals. Give five examples of non-metals.
(b) Name a non-metal which conducts electricity.
(c) Name a non-metal having lustre (shining surface).
(d) Name a non-metal which is extremely hard.
(e) How do non-metals react with oxygen? Explain with an example. Give equation of the reaction involved. What is the nature of the product formed? How will you demonstrate it?

Answer:

(a) Non-metals are elements that do not show metallic characteristics, such as conductivity of electricity and heat, malleability and ductility.They are non-lustrous, generally soft and non-sonorous. Hydrogen, nitrogen, oxygen, carbon and silicon are five examples of non-metals.

(b) Carbon in the form of graphite is a non-metal that conducts electricity.

(c) Iodine is a non-metal that is lustrous (shining surface).

(d) Carbon in the form of diamond is a non-metal, which is extremely hard.

(e) Non-metals react with oxygen to form acidic or neutral oxides. The oxide is covalent in nature as it is formed by the sharing of electrons.When carbon burns in air (oxygen), it forms an acidic oxide called carbon dioxide.

For example, the non-metal, carbon reacts with oxygen:
  C(s) +  O2(g) CO2(g)

The product formed is acidic in nature. It can be demonstrated by dissolving carbon dioxide in water to form carbonic acid. This resultant solution or carbonic acid, when added to blue litmus solution, changes the solution's colour to red. This shows that the oxide formed is acidic in nature.

Page No 135:

Question 64:

(a) What is meant by the reactivity series of metals? Arrange the following metals in an increasing order of their reactivities towards water:
Zinc, Iron, Magnesium, Sodium
(b) Hydrogen is not a metal but still it has been assigned a place in the reactivity series of metals. Why?
(c) Name one metal more reactive and another less reactive than hydrogen.
(d) Name one metal which displaces copper from copper sulphate solution and one which does not.
(e) Name one metal which displaces silver from silver nitrate solution and one which does not.

Answer:

(a) The arrangement of metals in a vertical column in the order of their decreasing reactivity is called the reactivity series of metals.
The increasing order of these metals according to their reactivity with water is:
 Iron < Zinc < Magnesium < Sodium

(b) Hydrogen is not a metal but it has been assigned a place in the reactivity series of metals because like metals, it also loses electrons and forms positive ions, H+.

(c) Lead is the metal that is more reactive than hydrogen and copper is the metal that is less reactive than hydrogen.

(d) Zinc is the metal that can displace copper from copper sulphate solution and silver is the metal that does not displace copper from copper sulphate solution.

(e) Copper is the metal that can displace silver from silver nitrate solution and gold is the metal that cannot displace silver from silver nitrate solution.

Page No 135:

Question 65:

(a) State any three differences between the physical properties of metals and non-metals.
(b) Differentiate between metals and non-metals on the basis of their chemical properties.
(c) State three reasons (of which at least one must be chemical) for believing that sodium is a metal.
(d) State three reasons (of which at least one must be chemical) for believing that sulphur is a non-metal.
(e) Which non-metal has been placed in the reactivity series of metals?

Answer:

(a) 

Physical properties Metal Non-metal
(i) Malleability and ductility Metals are malleable and ductile. Non-metals are not malleable and ductile.
(ii) Hard and tough Metals are hard and tough. Non-metals are soft.
(iii) Conductivity of heat and electricity Metals are good conductors of heat and electricity. Non-metals are bad conductors of heat and electricity.                   


(b)  
Chemical properties Metals Non-metals
(i) Reaction with oxygen Metals react with oxygen to form basic oxides. Non-metals react with oxygen to form acidic or neutral oxides.            
(ii) Reaction with water Metals react with water to form metal hydroxide and hydrogen gas Non-metals do not react with water.
(iii) Reaction with dilute acid Metals react with dilute acids to form metal salts and they displace hydrogen from acids. Non-metals do not react with dilute acids.

(c) (i) Sodium reacts violently with water to form sodium hydroxide and hydrogen gas, which is the property of a metal.
    (ii) Sodium reacts with dilute acids and displaces hydrogen gas from the acids and forms sodium salt, which is the property of a metal.
    (iii) Sodium exists in solid state at room temperature, which is the property of a metal.

(d) (i)  Sulphur is not lustrous, which shows that it is a non-metal.
     (ii)  Sulphur is a non-sonorous material, which shows that it is a non-metal.
     (iii) Sulphur reacts with oxygen to form sulphur dioxide that is acidic in nature. This shows that it is a non-metal.

(e) Hydrogen is the non-metal that has been placed in the reactivity series of metals because it loses electrons to form positive ions (H+) like metals.

Page No 135:

Question 66:

The elements whose oxides can turn phenolphthalein solution pink are:
(a) Na and K
(b) K and C
(c) Na and S
(d) K and P

Answer:

(a) Na and K
Because Na and K are metals whose oxides are basic in nature, which can turn phenolphthalein solution pink.

Page No 135:

Question 67:

"Is malleable and ductile". This best describes:
(a) a metal
(b) a compound
(c) a non-metal
(d) a solution

Answer:

(a) a metal
Because malleability and ductility are the properties of metals.

Page No 135:

Question 68:

One of the following is not a neutral oxide. This is:
(a) CO
(b) H2O
(c) N2O
(d) Na2O

Answer:

(d) Na2O
Because Na2O is a metal oxide that is basic in nature.

Page No 135:

Question 69:

A basic oxide will be formed by the element:
(a) K
(b) S
(c) P
(d) Kr

Answer:

(a) K
Because K is a metal, its oxide K2O is a basic oxide.

Page No 135:

Question 70:

An acidic oxide is produced by the element:
(a) Na
(b) C
(c) Ca
(d) H

Answer:

(b) C
Because C is a non-metal, its oxide CO2 is an acidic oxide.

Page No 135:

Question 71:

You are given a solution of AgNO3. Which of the following do you think cannot displace Ag from AgNO3 solution?
(a) Magnesium
(b) Zinc
(c) Gold
(d) Copper

Answer:

(c) Gold
Because gold is less reactive than silver. The other metals are more reactive than silver and can displace it from silver nitrate solution.

Page No 135:

Question 72:

Out of aluminium, copper, calcium and tin, the most reactive metal is:
(a) aluminium
(b) copper
(c) tin
(d) calcium

Answer:

(d) calcium
Calcium lies above the other three metals in reactivity series.

Page No 135:

Question 73:

The least reactive metal among the following is:
(a) sodium
(b) silver
(c) copper
(d) lead

Answer:

(b) silver
Because silver lies below other metals in the reactivity series.

Page No 135:

Question 74:

An element X reacts with hydrogen, when heated, to form a covalent hydride H2X. If H2X has a smell of rotten eggs, the element X is likely to be:
(a) carbon
(b) sulphur
(c) chlorine
(d) phosphorus

Answer:

(b) sulphur
Because sulphur reacts with hydrogen to form a covalent hydride, H2S that has the smell of rotten eggs.

Page No 135:

Question 75:

Out of the following oxides, the amphoteric oxide is:
(a) Fe2O3
(b) Al2O3
(c) P2O5
(d) N2O

Answer:

(b) Al2O3
Al2O3 can react with acids as well as bases to form salt and water.

Page No 135:

Question 76:

The metals which can produce amphoteric oxides are:
(a) sodium and aluminium
(b) zinc and potassium
(c) calcium and sodium
(d) aluminium and zinc

Answer:

(d) aluminium and zinc
Both aluminium and zinc oxides react with acids and bases to form salt and water.

Page No 135:

Question 77:

An element X forms two oxides XO and XO2. The oxide XO is neutral but XO2 is acidic in nature. The element X is most likely to be:
(a) sulphur
(b) carbon
(c) calcium
(d) hydrogen

Answer:

(b) carbon
Carbon forms two oxides, CO and CO2. The oxide CO is neutral and CO2 is acidic in nature.

Page No 135:

Question 78:

The elements whose oxides can turn litmus solution blue are:
(a) carbon and sulphur
(b) sodium and carbon
(c) potassium and magnesium
(d) magnesium and sulphur

Answer:

(c) potassium and magnesium
Potassium and magnesium are metals whose oxides are basic in nature. These oxides turn red litmus solution to blue.



Page No 136:

Question 79:

The elements whose oxides can turn litmus solution red are:
(a) lithium and sodium
(b) copper and potassium
(c) carbon and hydrogen
(d) phosphorus and sulphur

Answer:

(d) phosphorus and sulphur
Phosphorus and sulphur are non-metals whose oxides are acidic in nature. These oxides turn blue litmus solution to red.

Page No 136:

Question 80:

Zinc oxide is a metal oxide. Which of the following term best describes the nature of zinc oxide:
(a) an acidic oxide
(b) a basic oxide
(c) an amphoteric oxide
(d) a neutral oxide

Answer:

(c) an amphoteric oxide
Zinc oxide can react with both acids and bases to form salt and water.

Page No 136:

Question 81:

A metal less reactive and another metal more reactive than hydrogen are:
(a) aluminium and lead
(b) iron and magnesium
(c) copper and tin
(d) copper and mercury

Answer:

(c) copper and tin
Copper lies below hydrogen and tin lies above hydrogen in the reactivity series of metals.

Page No 136:

Question 82:

An element E reacts with water to form a solution which turns phenolphthalein solution pink. The element E is most likely to be:
(a) S
(b) Ca
(c) C
(d) Ag

Answer:

(b) Ca
Calcium is the metal that can react with water to form calcium hydroxide, which is basic in nature. Hence it turns phenolphthalein solution pink.

Page No 136:

Question 83:

An element reacts with oxygen to give a compound with a high melting point. This compound is also soluble in water. The element is likely to be:
(a) calcium
(b) carbon
(c) silicon
(d) iron

Answer:

(a) calcium
Calcium reacts with oxygen to form calcium oxide with a high melting point. This compound is also soluble in water.

Page No 136:

Question 84:

Which one of the following four metals would be displaced from the solution of its salt by the other three metals?
(a) Zn
(b) Ag
(c) Cu
(d) Mg

Answer:

(b) Ag
Ag will be replaced from the solution of its salt by the other three metals.

Page No 136:

Question 85:

An element is soft and can be cut with a knife. It is very reactive and cannot be kept open in the air. It reacts vigorously with water. The element is most likely to be:
(a) Mg
(b) S
(c) P
(d) Na

Answer:

(d) Na
Na is soft and can be cut with a knife. It is very reactive and cannot be kept in open air. It reacts vigorously with water.

Page No 136:

Question 86:

Which of the following metal exists in the liquid state?
(a) Na
(b) Ag
(c) Cr
(d) Hg

Answer:

(d) Hg
Mercury (Hg) is the metal that exists in liquid state at room temperature.

Page No 136:

Question 87:

Which of the following non-metal is a liquid?
(a) carbon
(b) sulphur
(c) bromine
(d) iodine

Answer:

(c) bromine
Bromine is the non-metal that is a liquid.

Page No 136:

Question 88:

Which of the following pair of reactants can undergo a displacement reaction under appropriate conditions?
(a) MgSO4 + Fe
(b) ZnSO4+ Fe
(c) MgSO4 + Pb
(d) CuSO4 + Fe

Answer:

(d) CuSO4 + Fe
Iron (Fe) is a more reactive metal than copper, which can displace copper from copper sulphate.

Page No 136:

Question 89:

An element E forms an oxide E2O. An aqueous solution of E2O turns red litmus paper blue.
(a) What is the nature of the oxide E2O?
(b) State whether element E is a metal or a non-metal.
(c) Give one example of an element like E.

Answer:

(a) The oxide E2O is basic in nature because it turns red litmus paper blue.

(b) The element E is a metal because metal oxides are basic in nature.

(c) Sodium (Na) is an element like E. It forms the oxide Na2O on reacting with oxygen, which turns red litmus paper blue.

Page No 136:

Question 90:

Metal A burns in air, on heating, to form an oxide A2O3 whereas another metal B burns in air only on strong heating to form an oxide BO. The two oxides A2O3 and BO can react with hydrochloric acid as well as sodium hydroxide solution to form the corresponding salts and water.
(a) What is the nature of oxide A2O3?
(b) What is the nature of oxide BO?
(c) Name one metal like A.
(b) Name one metal like B.

Answer:

(a) The oxide A2O3 is amphoteric, which can react with both acids and bases to form salt and water.

(b) The oxide BO is amphoteric, which can react with both acids and bases to form salt and water.

(c) Aluminium is a metal like A, which forms Al2O3 on reacting with oxygen.

(d) Zinc is a metal like B which forms ZnO on strongly heating in air (oxygen).

Page No 136:

Question 91:

An element X forms two oxides XO and XO2. The oxide XO has no action on litmus solution but oxide XO2 turns litmus solution red.
(a) What is the nature of oxide XO?
(b) What is the nature of oxide XO2?
(c) Would you call element X a metal or a non-metal? Give reason for your choice.
(d) Can you give an example of element like X?

Answer:

(a) The oxide XO is neutral in nature as it has no action on litmus solution.

(b) The oxide XO2 is acidic in nature as it turns litmus solution red.

(c) The element X is a non-metal because it forms an acidic and neutral oxide, which is the property of a non-metal.

(d) Carbon is a non-metal like the element X.

Page No 136:

Question 92:

State and explain the reaction, if any, of the following metals with a solution of copper sulphate:
(a) Gold
(b) Copper
(c) Zinc
(d) Mercury

Answer:

(a) Gold, being less reactive than copper, cannot displace copper from copper sulphate solution and hence, no reaction occurs.

(b) Copper reacts with copper sulphate to form copper and copper sulphate solution. So, practically no reaction occurs between them.

(c) Zinc is more reactive than copper and hence, displaces copper from its solution to form zinc sulphate solution and copper metal.
   
      Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)
 
(d) Mercury, being less reactive than copper, cannot displace copper from copper sulphate solution and hence, no reaction occurs.

Page No 136:

Question 93:

(a) Give the names and formulae of one metal chloride and one non-metal chloride.
(b) State an important property in which these metal chloride and non-metal chloride differ.
(c) Why do they differ in this property?

Answer:

(a) Sodium chloride is a metal chloride and its formula is NaCl.
     Hydrogen chloride is a non-metal chloride and its formula is HCl.

(b) Metal chlorides are ionic chlorides, which are electrolytes that can conduct electricity. However, non-metal chlorides are covalent chlorides, which are non-electrolytes that do not conduct electricity.

(c) They differ in this property because of the bonding in the chlorides. Metal chlorides have ionic bonds but non-metal chlorides have covalent bonds.



Page No 137:

Question 94:

In a solution of lead acetate, a strip of metal M was dipped. After some time, lead from the solution was deposited on the metal strip. Which metal is more reactive, M or lead?

Answer:

The metal M is more reactive than lead because M displaces lead from lead acetate solution, which can be done only by an element that is more reactive lead.

Page No 137:

Question 95:

CuSO4 (aq)  +  Fe (s)               FeSO4 (aq)  +  Cu (s)FeSO4 (aq)  +  Zn (s)               ZnSO4 (aq)  +  Fe (s)
On the basis of the above reactions, indicate which is most reactive and which is least reactive metal out of zinc, copper and iron.

Answer:

In the first reaction, Fe displaces Cu from CuSO4 solution. So Fe is more reactive than Cu.
In the second reaction, Zn displaces Fe from FeSO4 solution. So Zn is more reactive than Fe.
Hence it can be concluded that Zn is the most reactive metal and Cu is the least
reactive metal out of zinc, copper and iron.CuSO4 (aq)  +  Fe (s)               FeSO4 (aq)  +  Cu (s)

Page No 137:

Question 96:

Which of the following reactions will not occur? Why not?
(a) MgSO4 (aq)  +  Cu (s)                CuSO4 (aq) + Mg (s)
(b) CuSO4 (aq)  +  Fe (s)                FeSO4 (aq) + Cu (s)
(c) MgSO4 (aq)  +  Fe (s)                FeSO4 (aq) + Mg (s)

Answer:

Reactions (a) and (c) will not occur because copper is less reactive than magnesium. So, it cannot displace magnesium from its salt solution, the equation of which is shown in reaction (a).
Similarly, iron is less reactive than magnesium. So, it cannot displace magnesium from its salt solution, the equation of which is shown in reaction (c).

Page No 137:

Question 97:

In nature, metal A is found in a free state while metal B is found in the form of its compounds. Which of these two will be nearer to the top of the activity series of metals?

Answer:

The metal B will be nearer to the top of the activity series of metals because this metal is so reactive that it reacts with most elements to form its compounds. But metal A is less reactive as it exists in free state without reacting with other elements.

Page No 137:

Question 98:

If A, B, C, D, E, F, G, H, I, J and K represent metals in the decreasing order of their reactivity, which one of them is most likely to occur in a free state in nature?

Answer:

The metal K is most likely to occur in a free state in nature because it is the least reactive among other metals. Since it is the least reactive, it will not react with other elements.

Page No 137:

Question 99:

(a) Name a metal for each case:
(i) It does not react with cold as well as hot water but reacts with steam.
(ii) It does not react with any physical state of water?
(b) When calcium metal is added to water, the gas evolved does not catch fire but the same gas evolved on adding sodium metal to water catches fire. Why is it so?

Answer:

(a)  (i) Aluminium does not react with cold or hot water but it reacts with steam.
      (ii) Gold does not react with any physical state of water.

(b)  Sodium is more reactive than calcium. The reaction between sodium and water produces a large amount of heat, which causes the evolved hydrogen to burn and catch fire. But in the reaction between calcium and water, no heat is produced and hence the evolved hydrogen does not catch fire.

Page No 137:

Question 100:

A zinc plate was kept in a glass container having CuSO4 solution. On examining it was found that the blue colour of the solution is getting lighter and lighter. After a few days, when the zinc plate was taken out of the solution, a number of small holes were noticed in it. State the reason and give chemical equation of the reaction involved.

Answer:

Zinc is more reactive than copper and hence zinc displaces copper from copper sulphate solution to form zinc sulphate solution, which is of green colour, and copper. It is due to formation of the green zinc sulphate solution that the blue colour of copper sulphate gets lighter and lighter.
Zinc from the rod reacts with the solution to form zinc sulphate solution. So the plate has a number of small holes in it.
  Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)



Page No 167:

Question 1:

What is the name of the chemical bond formed:
(a) by the sharing of electrons between two atoms?
(b) by the transfer of electrons from one atom to another?

Answer:

(a) Covalent bond is formed by the sharing of electrons between two atoms.
(b) Ionic bond is formed by the transfer of electrons from one atom to another.

Page No 167:

Question 2:

Name a carbon containing molecule which has two double bonds.

Answer:

Carbon dioxide is a carbon containing molecule which has two double bonds. It is represented as O=C=O.

Page No 167:

Question 3:

What would be the electron-dot structure of carbon dioxide which has the formula CO2?

Answer:

 The electron dot structure of CO2 can be represented as:
 

                                     O.... :: C : O.... :                                     

Page No 167:

Question 4:

What type of chemical bonds is formed between:
(a) potassium and bromine?
(b) carbon and bromine?

Answer:

(a) Potassium is a metal and bromine is a non-metal. Hence the type of bond formed between potassium and bromine is ionic bond.

(b) Carbon and bromine are both non-metals. Hence the type of bond formed between them is covalent bond.

Page No 167:

Question 5:

(a) What do we call those particles which have more or less electrons than the normal atoms?
(b) What do we call  those particles which have more electrons than the normal atoms?
(c) What do we call those particles which have less electrons than the normal atoms?

Answer:

(a) The particles that have more or less electrons than normal atoms are called ions.
(b) The particles that have more electrons than normal atoms are called anions.
(c) The particles that have less electrons than normal atoms are called cations.

Page No 167:

Question 6:

(a) The atomic number of sodium is 11. What is the number of electrons in Na+?
(b) The atomic number of chlorine is 17. What is the number of electrons in Cl?

Answer:

(a) Na+ contains one less electron than the Na atom (11 electrons). So it contains 11 − 1 = 10 electrons.
(b) Cl  contains one more electron than the Cl atom (17 electrons). So it contains 17 + 1 = 18 electrons.

Page No 167:

Question 7:

The atomic number of an element X is 8 and that of element Y is 12. Write down the symbols of the ions you would expect to be formed from their atoms.

Answer:

Element X is oxygen (atomic number 8). Oxygen needs two more electrons to complete its noble gas configuration. The element Y is magnesium (atomic number 12). Magnesium has two extra electrons than are needed to achieve a stable configuration. Hence, magnesium donates two electrons to the oxygen atom to attain a stable configuration.
So, Mg becomes Mg2+ and O becomes O2- as ions.

Page No 167:

Question 8:

(a) Write down the electronic configuration of (i) magnesium atom, and (ii) magnesium ion.
(At. No. of Mg = 12)
(b) Write down the electronic configuration of (i) sulphur atom, and (ii) sulphide ion.
(At. No. of S = 16)

Answer:

(a) (i) The electronic configuration of a magnesium atom (atomic no. 12) is: K  L  M
                                                                                                                            2   8   2
      (ii) The electronic configuration of a magnesium ion (atomic no. 10) is: K   L
                                                                                                                         2   8
(b) (i) The electronic configuration of a sulphur atom (atomic no. 16) is: K  L  M
                                                                                                                    2   8   6
      (ii) The electronic configuration of a sulphide ion (atomic no. 18) is: K   L  M
                                                                                                                     2   8   8

Page No 167:

Question 9:

What type of chemical bonds are present in a solid compound which has a high melting point, does not conduct electricity in the solid state but becomes a good conductor in the molten state?

Answer:

In a solid compound that has a high melting point, that does not conduct electricity in the solid state but becomes a good conductor in the molten state, the chemical bonds between atoms will be ionic.

Page No 167:

Question 10:

State whether the following statement is true or false:
The aqueous solution of an ionic compound conducts electricity because there are plenty of free electrons in the solution.

Answer:

False. The aqueous solution of an ionic compound conducts electricity because there are plenty of free ions in the solution. (But not the free electrons.)

Page No 167:

Question 11:

What type of bonds are present in hydrogen chloride and oxygen?

Answer:

The chemical bond present in hydrogen chloride is a covalent bond. In hydrogen chloride, one electron each from hydrogen and chlorine are shared between the two atoms.
The chemical bond present in an oxygen molecule is covalent bond. In an oxygen molecule, each oxygen atom shares  one electron to complete its outermost shell.

Page No 167:

Question 12:

Write the electron-dot structures for the following molecules?
(i) NaCl
(ii) Cl2

Answer:

(i) The electron-dot structure of NaCl:

        
 

(ii) The electron-dot structure of Cl2:
         

Page No 167:

Question 13:

What type of bonds are present in water molecule? Draw the electron-dot structure of water (H2O).

Answer:

The chemical bond present in a water molecule is covalent bond because one oxygen atom shares its two electrons with two hydrogen atoms.

The electron-dot structure of a water molecule is represented as:
                 
                                                                              

Page No 167:

Question 14:

What type of bonds are present in methane (CH4) and sodium chloride (NaCl)?

Answer:

The chemical bond present in methane (CH4) is covalent bond as a carbon atom shares its four valence electrons with four hydrogen atoms to form one methane molecule.
The chemical bond present in sodium chloride (NaCl) is ionic bond as a sodium atom transfers one electron to a chlorine atom to form the ionic bond in NaCl.

Page No 167:

Question 15:

State one major difference between covalent and ionic bonds and give one example each of covalent and ionic compounds.

Answer:

Covalent compounds have usually low melting points and boiling points. But ionic compounds have high melting points and boiling points.
An example for a covalent compound is methane (CH4).
An example for an ionic compound is sodium chloride (NaCl).

Page No 167:

Question 16:

What type of bonds are present in the following molecules? Draw their electron-dot structures.
(i) H2
(ii) CH4
(iii) Cl2
(iv) CO2

Answer:

(i) Covalent bond is the chemical bond present in an H2molecule as one hydrogen atom shares its electrons with another hydrogen atom.
    The electron-dot structure is:
      

(ii) Covalent bond is the chemical bond present in CH4 as a carbon atom shares its electrons with four hydrogen atoms.
     The electron-dot structure is:
      

(iii) Covalent bond is the chemical bond present in a Cl2molecule as one chlorine atom shares its electrons with another chlorine atom.
       The electron-dot structure is:
        

(iv) Covalent bond is the bond present in O2 as one oxygen atom shares its electrons with another oxygen atom.
      The electron-dot structure is:
         

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Question 17:

Which inert gas electron configuration do the Cl atoms in Cl2 molecule resemble? What is this electron configuration?

Answer:

Argon is the inert gas that resembles the Cl atoms in electronic configuration in a Cl2 molecule. This electronic configuration is  K L M.
                                                    2
  8 8

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Question 18:

Which of the following compounds are ionic and which are covalent?
Urea, Cane sugar, Hydrogen chloride, Sodium chloride, Ammonium chloride, Carbon tetrachloride, Ammonia, Alcohol, Magnesium chloride.

Answer:

The ionic compounds are sodium chloride, ammonium chloride and magnesium chloride. The covalent compounds are urea, cane sugar, hydrogen chloride, carbon tetrachloride, ammonia and alcohol.

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Question 19:

Give one example each of the following:
(i) A molecule containing a single covalent bond
(ii) A molecule containing a double covalent bond
(iii) A molecule containing a triple covalent bond
(iv) A compound containing an ionic bond

Answer:

(i)  A molecule containing a single covalent bond: hydrogen molecule (H − H)
(ii) A molecule containing a double covalent bond: oxygen molecule (O=O)
(iii) A molecule containing a triple covalent bond: nitrogen molecule (NN)
(iv) A compound containing an ionic bond: sodium chloride (NaCl)



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Question 21:

(a) What is a covalent bond? What type of bond exists in (i) CCl4, and (ii) CaCl2?
(b) What is an ionic bond? What type of bond is present in oxygen molecule?

Answer:

(a) The chemical bond that is formed between two atoms by the sharing of electrons is known as a covalent bond.
      (i) A covalent bond exists in CCl4.
      (ii) An ionic bond exists in CaCl2.
(b) The chemical bond that is formed by the transfer of electrons between two atoms is known as an ionic bond.
      Covalent bond is the chemical bond present in an oxygen molecule.

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Question 22:

(a) What is an ion? Explain with examples.
(b) What is the nature of charge on (i) a cation, and (ii) an anion?
(c) Name the cation and anion present in MgCl2. Also write their symbols.

Answer:

(a) An electrically-charged atom (or group of atoms) is called an ion. it is formed by the loss or gain of an electron by an atom.
Ions are of two types: cations and anions.
For example: A sodium ion (Na+) is formed when a sodium atom loses one electron. A sodium ion is a cation.
A chloride ion (Cl-) is formed when a chlorine atom gains one electron. A chloride ion is an anion.

(b) (i) The charge on a cation is positive.
     (ii) The charge on an anion is negative.

(c) The cation present in MgCl2 is a magnesium ion that is represented as Mg2+ and the anion present is a chloride ion that is represented by Cl-.

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Question 23:

(a) What type of chemical bond is present in chlorine molecule? Explain your answer.
(b) Explain the formation of a chlorine molecule on the basis of electronic theory of valency.

Answer:

(a) In a chlorine molecule, the atoms bonded by a covalent bond as a chlorine atom needs one electron to complete its octet and shares one electron with another chlorine atom.

(b) The atomic number of chlorine is 17 and its electronic configuration is 2, 8, 7. It needs one more electron to complete its octet. Chlorine gets this electron by sharing with another atom. So each chlorine atom shares one electron to form a chlorine molecule.

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Question 24:

(a) Giving one example each, state what are (i) ionic compounds, and (ii) covalent compounds.
(b) Compare the properties of ionic compounds and covalent compounds.

Answer:

(a) (i) Ionic compounds are compounds that are formed by the transfer of electrons by an atom to another atom.
      Example: Sodium chloride (NaCl) is an ionic compound, where sodium transfers one electron to chlorine atom.
     
    (ii) Covalent compounds are compounds that are formed by the sharing of electrons between two or more atoms.
      Example: Hydrogen (H2) is a covalent compound, where each hydrogen atom shares one electron to form a stable compound.

(b)

Ionic compound Covalent compound
Ionic compounds are usually crystalline solids. Covalent compounds are usually liquids or gases.
Ionic compounds have high melting and boiling points. Covalent compounds have low melting and boiling points.
Ionic compounds are usually soluble in water. Covalent compounds are usually insoluble in water.
Ionic compounds conduct electricity when dissolved in water or melted. Covalent compounds do not conduct electricity.
Ionic compounds are insoluble in organic solvents. Covalent compounds are soluble in organic solvents.

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Question 25:

Explain why:
(a) covalent compounds have generally high melting points.
(b) ionic compounds have generally high melting points.

Answer:

(a) Covalent compounds are made up of electrically neutral molecules. So the force of attraction between the molecules is very weak. Only a small amount of heat energy is required to break these weak molecular forces, due to which covalent compounds have a low melting point.
(b) Ionic compounds have generally high melting points because ionic compounds are composed of positively and negatively charged ions. So there is a strong force of attraction between these ions. A large amount of heat energy is required to break this strong force of attraction and hence they have a high melting point.

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Question 26:

(a) Give two general properties of ionic compounds and two those of covalent compounds.
(b) State one test by which sodium chloride can be distinguished from sugar.

Answer:

(a)

Ionic Compounds Covalent Compounds
Ionic compounds usually have high melting and boiling points. Covalent compounds usually have low melting and boiling points.
Ionic compounds generally conduct electricity when dissolved in water or in a molten state. Covalent compounds usually do not conduct electricity.

(b) Sodium chloride and sugar can be distinguished by the electrical conductivity test. Sugar, being a covalent compound, does not conduct electricity when dissolved in water but sodium chloride when dissolved in water conducts electricity.

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Question 27:

(a) Explain why, ionic compounds conduct electricity in solution whereas covalent compounds do not conduct electricity.
(b) Which of the following will conduct electricity and which not?
MgCl2, CCl4, NaCl, CS2, Na2S
Give reasons for your choice.

Answer:

(a) An ionic compound consists of ions held by strong forces in solid state. But when an ionic compound dissolves in water to form a solution, the constituent ions get separated and they disperse across the solution. This helps in conducting an electric current through the solution. Covalent compounds are formed by the sharing of electrons. So ions that conduct electricity are not involved in these compounds. Hence covalent compounds do not conduct electricity.

(b) The compounds that conduct electricity are MgCl2, NaCl and Na2S. These compounds are ionic in nature So conduct electricity. The compounds that do not conduct electricity are CS2 and CCl4. These compounds are covalent in nature. Hence they do not conduct electricity.

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Question 28:

(a) Name one ionic compound containing chlorine and one covalent compound containing chlorine.
(b) How will you find out which of the water soluble compound A or B is ionic?

Answer:

(a) An ionic compound that contains chlorine is sodium chloride (NaCl) and a covalent compound that contains chlorine is hydrogen chloride (HCl).

(b) The test to find out which water soluble compound, A or B, is ionic can be done by the electrical conductivity test. The compound solution that conducts electricity must be ionic in nature and the other compound solution that does not conduct electricity must be covalent in nature.

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Question 29:

Explain why, a solution of cane sugar does not conduct electricity but a solution of common salt is a good conductor of electricity.

Answer:

A solution of cane sugar does not conduct electricity because cane sugar is a covalent compound, which is bonded by the sharing of electrons. In the solution, it does not have ions to conduct electrical charges across the solution. But common salt consists of sodium and chloride ions and is formed by the transfer of one electron of sodium to a chlorine atom. When this salt dissolves in water to form its solution, ions get dispersed across the solution due to which the solution conducts electricity across the solution.

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Question 30:

Give the formulae of the compounds that would be formed by the combination of the following pairs of elements:
(a) Mg and N2
(b) Li and O2
(c) Al and Cl2
(d) K and H

Answer:

(a) Mg3N2
(b) Li2O
(c) AlCl3
(d) KH

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Question 31:

(a) What are noble gases? What is the characteristic of the electronic configuration of noble gases?
(b) What is the cause of chemical bonding (or chemical combination) of atoms of elements?

Answer:

(a) The elements in group 18 of the periodic table, which do not combine with other elements are known as noble gases. These elements are helium, neon, argon, krypton, xenon and radon. The important characteristic of the electronic configuration of the noble gases is that they have completely filled outermost shells or valence shells, which is the reason for the inertness of these elements.

(b) The atoms of elements undergo chemical bonding (or chemical combination) to attain the noble gas electronic configuration and become more stable. By chemical bonding, atoms share or transfer their electrons to make their outermost shells completely filled and thus attain stability.

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Question 32:

(i) Write electron-dot structures for magnesium and oxygen.
(ii) Show the formation of MgO by the transfer of electrons.
(iii) What are the ions present in this compound?

Answer:

(i) The electron-dot structure of magnesium and oxygen is:
          


(ii)
  

(iii) In this compound, a magnesium ion (Mg2+) and an oxide ion (O2-) are present as a cation and an anion, respectively.                                                                      

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Question 33:

Draw the electron-dot structure of a hydrogen chloride molecule:
(i) Which inert gas does the H atom in HCl resemble in electron arrangement?
(ii) Which inert gas does the Cl atom in HCl resemble in electron arrangement?

Answer:

The electron-dot structure of a hydrogen chloride molecule is:
              
It is formed by sharing one electron by the hydrogen and chlorine atoms.

(i) The H atom in HCl resembles helium gas in its electron arrangement by sharing one electron with the chlorine atom to get two electrons in its outermost shell.

(ii) The Cl atom in HCl resembles argon gas in its electron arrangement by sharing one electron with the hydrogen atom to get eight electrons in its outermost shell. Its electronic configuration becomes 2, 8, 8.

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Question 34:

What type of bonding would you expect between the following pairs of elements?
(i) Calcium and Oxygen
(ii) Carbon and Chlorine
(iii) Hydrogen and Chlorine

Answer:

(i) Calcium and oxygen react to form calcium oxide (CaO) by ionic bonding. Calcium contains two electrons in its outermost shell and oxygen contains six electrons in its outermost shell. So the calcium atom transfers two electrons to the oxygen atom to attain the noble gas configuration for both atoms.

(ii) Carbon and chlorine react to form carbon tetrachloride (CCl4) by covalent bonding. Carbon contains four electrons in its outermost shell and chlorine contains seven electron in its outermost shell. So carbon shares electrons electrons with four chlorine atoms (one electron from each chlorine atom) to attain stability.

(iii) Hydrogen and chlorine react to form (HCl) by covalent bonding. Hydrogen contains one electron in its outermost shell and chlorine also contains one electron in its outermost shell. So the hydrogen and chlorine atoms share one electron from each atom to attain stability.

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Question 20:

Fill in the blanks in the following sentences:
(i) Two atoms of the same element combine to form a molecule. The bond between them is known as ................ bond.
(ii) Two chlorine atoms combine to form a molecule. The bond between them is known as ..........
(iii) In forming oxygen molecule, .............. electrons are shared by each atom of oxygen.
(iv) In forming N2 molecule, .............. electrons are shared by each atom of nitrogen.
(v) The number of single covalent bonds in C2H2 molecule are ...........
(vi) Melting points and boiling points of ionic compounds are generally ............ than those of covalent compounds.

Answer:

(i) Two atoms of the same element combine to form a molecule. The bond between them is known as a covalent bond.

(ii) Two chlorine atoms combine to form a molecule. The bond between them is known as a covalent bond.

(iii) In forming an oxygen molecule, two electrons are shared by each atom of oxygen.

(iv) In forming an N2 molecule, three electrons are shared by each atom of nitrogen.

(v) The number of single covalent bonds in a C2H2 molecule is two.

(vi) Melting points and boiling points of ionic compounds are generally higher than those of covalent compounds.



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Question 35:

Describe how sodium and chlorine atoms are changed into ions when they react with each other to form sodium chloride, NaCl. What is the name given to this type of bonding? (At. No of sodium = 11; At. No. of chlorine = 17)

Answer:

The electronic configuration of sodium is 2, 8, 1. The outermost shell contains one electron. For the atom to be stable, it should lose one electron and thus attain the noble gas configuration of 2, 8.

Na - e- → Na+
The electronic configuration of the chlorine atom is 2, 8, 7. The outermost shell contains seven electrons. For the atom to be stable, it requires one electron and thus attain the noble gas configuration of 2, 8, 8.
Cl + e- →  Cl-   

Hence sodium donates one electron to the chlorine atom when they react with each other to form a stable compound, sodium chloride, NaCl. This type of bonding that takes place through transfer of electrons is known as ionic bonding.

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Question 36:

What is the difference between a cation and an anion? How are they formed? Give the names and symbols of one cation and one anion.

Answer:

Cation Anion
The ion that is positively charged or that loses its electrons is known as a cation. The ion that is positively charged or that gains electrons is known as an anion.

Cations are formed when an atom donates its electrons to another atom to attain the noble gas configuration and become stable.
Anions are formed when an atom gains electrons from another atom to complete its outermost shell or attain the noble gas configuration and become stable.

An example of a cation is a sodium ion (Na+) that is formed by losing one electron.
An example of an anion is a chloride ion (Cl-) that is formed by gaining one electron.

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Question 37:

Using electron-dot diagrams which show only the outermost shell electrons, show how a molecule of nitrogen, N2, is formed from two nitrogen atoms. What name is given to this type of bonding? (Atomic number of nitrogen is 7)

Answer:

The atomic number of nitrogen is 7. Its electronic configuration is 2, 5. Hence, it contains five electrons in its outermost shell.

        
Three electrons from each nitrogen atom are shared to complete its octet. This sharing of electrons results in covalent bonding in a nitrogen molecule.

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Question 38:

Draw the electron-dot structures of the following compounds and state the type of bonding in each case:
(i) CO2
(ii) MgO
(iii) H2O
(iv) HCl
(v) MgCl2

Answer:

(i) The electron-dot structure of CO2 is:
     
Both oxygen and carbon share their two electrons to form a covalent bond in carbon dioxide, CO2.

(ii) The electron-dot structure of MgO is:
          Mg2+:O:....2-  
Magnesium donate its two electrons to the oxygen atom to form an ionic bond in magnesium oxide, MgO .


(iii) The electron-dot structure of H2O is: 
      
The oxygen atom shares its one electron with each hydrogen atom to form a covalent bond in the water molecule.

(iv) The electron-dot structure of HCl is:
        H:Cl....:  
The hydrogen atom shares its one electron with a chlorine atom to form a covalent bond.

(v) The electron-dot structure of MgCl2 is:
      
The magnesium atom transfers its one electron to each chlorine atom to form an ionic bond.

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Question 39:

Using electron-dot diagrams which show only the outermost shell electrons, show how a molecule of oxygen, O2, is formed from two oxygen atoms. What name is given to this type of bonding? (At. No. of oxygen = 8)

Answer:

The atomic number of oxygen is 8. Its electronic configuration is 2, 6. Hence it has six electrons in its outermost shell.

In an oxygen molecule, one oxygen atom shares its two electrons with another oxygen atom. Hence this type of bonding is called covalent bonding.

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Question 40:

Draw the electron-dot structures of the following compound and state the type of bonding in each case:
(i) KCl
(ii) NH3
(iii) CaO
(iv) N2
(v) CaCl2

Answer:

(i) The electron-dot structure of KCl is:

In KCl, the potassium atom transfers its 1 electron to the chlorine atom to form an ionic bond.

(ii) The electron-dot structure of NH3 is:

In NH3, each hydrogen atom shares its one electron with the nitrogen atom to form a covalent bond.

(iii) The electron-dot structure of CaO is:

In CaO, the calcium atom transfers its two electrons to the  oxygen atom to form an ionic bond.

(iv) Electron-dot structure of N2 is
     
In N2, one nitrogen atom shares its three electrons with another nitrogen atom to form a covalent bond.

(v) The electron-dot structure of CaCl2 is:
    
In CaCl2, one calcium atom transfers its two electrons to the two chlorine atoms to form an ionic bond.

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Question 41:

Explain why, a salt which does not conduct electricity in the solid state becomes a good conductor in molten state.

Answer:

The ions of salt in solid state are bonded with strong inter-particle forces. Hence, they are not free to move in solid state and cannot conduct electricity. However, when this salt melts, its constituent ions get separated from each other. Thus, they are free to move across the melted salt and conduct electricity.

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Question 42:

(a) Write down the electronic configuration of (i) sodium atom, and (ii) chlorine atom.
(b) How many electrons are there in the outermost shell of (i) a sodium atom, and (ii) a chlorine atom?
(c) Show the formation of NaCl from sodium and chlorine atoms by the transfer of electron(s).
(d) Why has sodium chloride a high melting point?
(e) Name the anode and the cathode used in the electrolytic refining of impure copper metal.

Answer:

(a)  (i) Atomic number of sodium is 11.
           Its electric configuration is K L M2  8  1.
      (ii) Atomic number of chlorine is 17.
           Its electronic configuration is K L M2  8  7.
(b) (i)  In a sodium atom, the number of electrons present in its outermost shell (M shell)  is one.
     (ii)  In a chlorine atom, the number of electrons present in its outermost shell (M shell)  is seven.

(c) Sodium and chlorine contain one and seven electrons, respectively, in their outermost shells. Hence, sodium transfers its one electron to chlorine atom and their octets get stabilised.
                             Na.:Cl:...Na+:Cl....:-  or NaCl

         
In a sodium chloride molecule, sodium atom transfers its one electron to chlorine atom to form an ionic bond.

(d) Sodium chloride contains positively charged sodium atom and negatively charged chlorine atom. Due to these oppositely charged species, there is a strong intermolecular force between them. Hence, it requires huge amount of energy to break this strong intermolecular force and melt the salt. Therefore, sodium chloride has a high melting point.

(e) In the electrolytic refining of impure copper metal, anode is the electrode at which oxidation takes place and cathode is the electrode at which reduction takes place.
Hence, at anode, a thick block of impure copper metal is used from which pure metal oxidises to copper ion in the solution.
At cathode, a thin strip of pure copper metal is used where copper ions from the solution reduce to copper metal and get deposited on the strip.

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Question 43:

(a) Write the electron arrangement in (i) a magnesium atom, and (ii) an oxygen atom.
(b) How many electrons are there in the valence shell of (i) a magnesium atom, and (ii) an oxygen atom?
(c) Show on a diagram the transfer of electrons between the atoms in the formation of MgO.
(d) Name the solvent in which ionic compounds are generally soluble.
(e) Why are aqueous solution of ionic compounds able to conduct electricity?

Answer:

(a)  (i) Magnesium atom contains 12 electrons.
          Its electron arrangement can be written as 2 K8 L2 M.
      (ii) Oxygen atom contains eight electrons.
           Its electron arrangement can be written as 2 K6 L.

(b)  (i) The valence shell of a magnesium atom contains two electrons.
      (ii) The valence shell of an oxygen atom contains six electrons.

(c)  


In a magnesium oxide molecule, magnesium atom transfers its two electrons to oxygen atom to form ionic bond.

(d) The solvent in which ionic compounds are generally soluble is water.

(e) Aqueous solution of ionic compounds is able to conduct electricity because in aqueous solution, the strong force of attraction between ions is vanished. Their constituent ions get separated and become free to move across the solution. Due to this movement of ions, aqueous solution of ionic compounds conducts electricity.

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Question 44:

(a) What is the electronic configuration of (i) a sodium atom, and (ii) an oxygen atom?
(b) What is the number of outermost electrons in (i) a sodium atom, and (ii) an oxygen atom?
(c) Show the formation of Na2O by the transfer of electrons between the combining atoms.
(d) Why are ionic compounds usually hard.
(e) How is it that ionic compounds in the solid state do not conduct electricity but they do so when in molten state?

Answer:

(a) 
(i) Sodium atom contains 11 electrons.
     Its electronic configuration can be written as 2K 8L 1M .
     
(ii) Oxygen atom contains eight electrons.
      Its electron arrangement can be written as 2K 6L .

(b)  (i) Sodium atom contains one electron in its outermost shell.
      (ii) Oxygen atom contains six electrons in its outermost shell.

(c) In sodium oxide molecule, two sodium atoms transfer their one electron to one oxygen atom to form ionic bonds.
     

(d) Ionic compounds are generally hard because they consist of oppositely-charged ions. These ions are strongly attracted to each other and form a hard crystalline solid.

(e) The ionic compounds in solid state are bonded with strong intermolecular forces and are not free to move. Therefore, they cannot conduct electricity. However, when this salt melts, its constituent ions are separated from each other and become free to move across the melted salt that can conduct electricity.

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Question 45:

(a) Write down the electron arrangement in (i) a magnesium atom, and (ii) a chlorine atom.
(b) How many electrons are there in the valence shell of (i) a magnesium atom, and (ii) a chlorine atom?
(c) Show the formation of magnesium chloride from magnesium and chlorine by the transfer of electrons.
(d) State whether magnesium chloride will conduct electricity or not. Give reason for your answer.
(e) Why are covalent compounds generally poor conductors of electricity?

Answer:

(a)  (i) Magnesium atom contains 12 electrons.
       Its electron arrangement can be written as 2K 8L 2M.
      (ii) Chlorine atom contains 17 electrons.
       Its electron arrangement can be written as 2K 8L 7M .

(b) (i) Magnesium atom in its valence shell contains two electrons.
      (ii) Chlorine atom in its valence shell contains seven electrons.

(c) In magnesium chloride molecule, magnesium atom transfers its two electrons to two chlorine atoms to form ionic bonds.


(d) Magnesium chloride will conduct electricity because it is an ionic compound. On melting or dissolving in water, it conducts electricity due to the separation of charged ions, which move freely across the solution.

(e) Covalent compounds are generally poor conductors of electricity because they do not contain ions that can move freely and pass electricity.

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Question 46:

The atomic number of an element X is 19. The number of electrons in its ion X+ will be:
(a) 18
(b) 19
(c) 20
(d) 21

Answer:

(a) 18
Because X+ ion contains one less electron than X.

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Question 47:

The atomic number of an element Y is 17. The number of electrons in its ion Y will be:
(a) 17
(b) 18
(c) 19
(d) 20

Answer:

(b) 18
Anion Y- has one more electron than its neutral atom. Y atom has 17 electrons and therefore, in Y-, number of electrons will be 18 (17 + 1).

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Question 48:

The atomic numbers of four elements A, B, C and D are 6, 8, 10 and 12 respectively. The two elements which can react to form ionic bonds (or ionic compound) are:
(a) A and D
(b) B and C
(c) A and C
(d) B and D

Answer:

(d) B and D

Element B has the electronic configuration of 2,6. It requires 2 electrons to complete its octet. On the other hand, element D has the electronic configuration of 2,8,2. It requires to release 2 electrons to complete its octet. Thus, both atoms react to form ionic bonds by transfer of electrons.

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Question 49:

The atomic numbers of four elements P, Q, R and S are 6, 10, 12 and 17 respectively. Which two elements can combine to form a covalent compound?
(a) P and R
(b) Q and S
(c) P and S
(d) R and S

Answer:

(c) P and S

Element P needs 4 electrons to complete its octet. They can be shared with element S, which needs only 1 electron to complete its octet.



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Question 50:

The solution of one of the following compounds will not conduct electricity. This compounds is:
(a) NaCl
(b) CCl4
(c) MgCl2
(d) CaCl2

Answer:

(b) CCl4
Because it is a covalent compound.

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Question 51:

The electronic configurations of three elements X, Y and Z are:

X : 2 Y : 2, 8, 7 Z : 2, 8, 2
Which of the following is correct regarding these elements?
(a) X is a metal
(b) Y is a metal
(c) Z is a non-metal
(d) Y is a non-metal and Z is a metal

Answer:

(d) Y is a non-metal and Z is a metal.

Element Y has 1 less electron to complete its octet which is a property of non-metals. On the other hand, element â€‹Z has 2 extra electrons to become stabilised which is a property of metals.

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Question 52:

Which one of the following property is generally not exhibited by ionic compounds?
(a) solubility in water
(b) electrical conductivity in solid state
(c) high melting and boiling points
(d) electrical conductivity in molten state

Answer:

(b) electrical conductivity in solid state

Because ionic compounds conduct electricity only in molten state or when dissolved in water to form aqueous solution.

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Question 53:

The electrons present in the valence shell of a noble gas atom can be:
(a) 8 only
(b) 2 only
(c) 8 or 2
(d) 8 or 4

Answer:

(c) 8 or 2

Helium is the only noble gas that has 2 electrons in its valence shell. Other noble gases have 8 electrons in their valence shells.

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Question 54:

The atomic number of an element X is 16. The symbol of ion formed by an atom of this element will be:
(a) X2+
(b) X3+
(c) X2
(d) X

Answer:

(c) X2
Element X needs 2 electrons to complete its octet. Therefore, it forms X2 anion.

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Question 55:

The number of protons in the nucleus of one atom of an element Y is 5. The symbol of ion formed by an atom of this element will be:
(a) Y3
(b) Y2+
(c) Y2−
(d) Y3+

Answer:

(d) Y3+
Number of protons is equal to number of electrons in an atom. Hence, electronic configuration of element Y is 2,3. Thus, it becomes stabilised by donating its 3 electrons to form Y3+.

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Question 56:

Out of KCl, HCl, CCl4 and NaCl, the compounds which are not ionic are:
(a) KCl and HCl
(b) HCl and CCl4
(c) CCl4 and NaCl
(d) KCl and CCl4

Answer:

(b) HCl and CCl4
Both these compounds are formed by sharing of electrons and hence, they form covalent bonds and not ionic bonds.

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Question 57:

Element X reacts with element Y to form a compound Z. During the formation of compound Z, atoms of X lose one electron each whereas atoms of Y gain one electron each. Which of the following property is not shown by compound Z?
(a) high melting point
(b) low melting point
(c) occurrence as solid
(d) conduction of electricity in molten state

Answer:

(b) low melting point
Compound Z is an ionic compound formed by transfer of electrons. Ionic compounds have high melting points.

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Question 58:

One of the following compounds is not ionic in nature. This compound is:
(a) Lithium chloride
(b) Ammonium chloride
(c) Calcium chloride
(d) Carbon tetrachloride

Answer:

(d) Carbon tetrachloride
It is a covalent compound.

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Question 59:

The rechargeable battery used in a mobile phone hand set is usually:
(a) lead ion battery
(b) sodium ion battery
(c) hydrogen ion battery
(d) lithium ion battery

Answer:

(d) lithium ion battery
Because lithium ion is a good conductor of electricity.

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Question 60:

The number of protons in one atom of an element X is 8. What will be the number of electrons in its ion X2?
(a) 8
(b) 9
(c) 10
(d) 11

Answer:

(c) 10
Number of protons is equal to the number of electrons in an atom. It is given that one atom of an element X contains 8 electrons.
Therefore, number of electrons in its ion X2 = 10 + 2 = 12

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Question 61:

If the number of protons in one atom of an element Y is 20, then the number of electrons in its ion Y2+ will be:
(a) 20
(b) 19
(c) 18
(d) 16

Answer:

(c) 18
Number of protons is equal to the number of electrons in an atom. It is given that an atom of an element Y contains 20 electrons. Therefore, number of electrons in its ion Y2+ is 18 (20 - 2).

Page No 170:

Question 62:

The noble gas having only two electrons in its valence shell is:
(a) Ar
(b) Ne
(c) He
(d) Kr

Answer:

(c) He
Atomic number of helium is 2. It contains 2 electrons in its atom.

Page No 170:

Question 63:

A covalent molecule having a double bond between its atoms is:
(a) Hydrogen
(b) Oxygen
(c) water
(d) ammonia

Answer:

(b) Oxygen
In an oxygen molecule, each oxygen atom shares its 2 electrons to complete its octet and forms double covalent bond between them.

Page No 170:

Question 64:

The molecules having triple bond in them are:
(a) oxygen and ethyne
(b) carbon dioxide and ammonia
(c) methane and ethene
(d) nitrogen and ethyne

Answer:

(d) nitrogen and ethyne
In a nitrogen molecule, each nitrogen atom shares its 3 electrons to form a triple bond among them. In an ethyne molecule, each carbon atom shares 3 electrons to form a triple bond among them.

Page No 170:

Question 65:

One of the following contains a double bond as well as single bonds. This is:
(a) CO2
(b) O2
(c) C2H4
(d) C2H2

Answer:

(c) C2H4
An ethene molecule (C2H4) contains one double bond between two carbon atoms and four single bonds between carbon and hydrogen atoms.

Page No 170:

Question 66:

Which of the following has a triple bond as well as single bonds?
(a) ethene
(b) methane
(c) ethyne
(d) nitrogen

Answer:

(c) ethyne
An ethyne molecule ( H-CC-H ) contains a triple bond between two carbon atoms and two single bonds between carbon and hydrogen atoms.



Page No 171:

Question 67:

Two non-metals combine with each other by the sharing of electrons to form a compound X.
(a) What type of chemical bond is present in X?
(b) State whether X will have a high melting point or low melting point.
(c) Will it be a good conductor of electricity or not?
(d) Will it dissolve in an organic solvent or not?

Answer:

(a) Covalent bond is present compound X because it is formed by sharing of electrons.
(b) Compound X has a low melting point because it is a covalent compound.
(c) Being a covalent compound, it will not be a good conductor of electricity.
(d) Covalent compounds dissolve in an organic solvent. Since X is a covalent compound, it will dissolve in an organic solvent.

Page No 171:

Question 68:

A metal combines with a non-metal by the transfer of electrons to form a compound Y.
(i) State the type of bonds in Y.
(ii) What can you say about its melting point and boiling point?
(iii) Will it be a good conductor of electricity?
(iv) Will it dissolve in an organic solvent or not?

Answer:

(i) Compound Y has ionic bonds because it is formed by transfer of electrons.
(ii) Its melting and boiling points will be high because of the strong intermolecular forces between them.
(iii) It will be a good conductor of electricity because of the presence of ions in it.
(iv) It will not be dissolved in an organic solvent because of its ionic nature.

Page No 171:

Question 69:

The electronic configurations of three elements X, Y and Z are as follows:

X 2, 4
Y 2, 7
Z 2, 1
(a) Which two elements will combine to form an ionic compound?
(b) Which two elements will react to form a covalent compound?
Give reasons for your choice.

Answer:

(a) Elements Y and Z combine to form an ionic compound by transfer of one electron from Z to Y.
(b) Elements X and Y will react to form a covalent compound because atom X can react with 4 atoms of Y to share their one electron.

Page No 171:

Question 70:

An element A has 4 valence electrons in its atom whereas element B has only one valence electron in its atom. The compound formed by A and B does not conduct electricity. What is the nature of chemical bond in the compound formed? Give its electron-dot structure.

Answer:

The compound formed by elements A and B has covalent bonds between their atoms. One atom of element A reacts with four atoms of element B to share its one electron.
Its electron-dot structure is represented as:

Page No 171:

Question 71:

In the formation of a compound XY2 atom X gives one electron to each Y atom. What is the nature of bond in XY2? Give two properties of XY2.

Answer:

Compound XY2 has ionic bonds in nature because it is formed by transfer of electrons.
Two properties of compound XY2are:

  1. It has high melting and boiling points.
  2. It conducts electricity in molten state or when dissolved in water (but not in solid state).

Page No 171:

Question 72:

An element 'A' has two electrons in the outermost shell of its atom and combines with an element 'B' having seven electrons in the outermost shell, forming the compound AB2. The compound when dissolved in water conducts electric current. Giving reasons, state the nature of chemical bond in the compound.

Answer:

Compound AB2 is ionic in nature.
Explanation: Atom A has two electrons in its outermost shell and atom B needs one electron to complete its octet. Atom A transfers its two electrons to two atoms of B (one electron to each atom B) and gets stabilised by forming ionic bonds.
When dissolved in water, the ions of this ionic compound get separated from each other and conduct electricity across the solution.

Page No 171:

Question 73:

The electronic configurations of two elements A and B are given below:

A 2, 6
B 2, 8, 1
(a) What type of chemical bond is formed between the two atoms of A?
(b) What type of chemical bond will be formed between the atoms of A and B?

Answer:

(a) Two atoms of A can form covalent bond between them by sharing two electrons from each atom. This allows them to complete their octets and get stabilised. The molecule so formed is A=A.
(b) Atoms A and B may react to form ionic bond by transfer of two electrons from two atoms of B to form an ionic compound AB2.

Page No 171:

Question 74:

Four elements A, B, C and D have the following electron arrangements in their atoms:

A 2, 8, 6
B 2, 8, 8
C 2, 8, 8, 1
D 2, 7
(a) What type of bond is formed when element C combines with element D?
(b) Which element is an inert gas?
(c) What will be the formula of the compound between A and C?

Answer:

(a)  When element C combines with element D, an ionic bond is formed between them by transfer of one electron from element C atom to element D atom.

(b) Element B is an inert gas because it has 8 electrons in its valence shell; i.e, it contains completely filled valence shell.

(c) Formula of the compound will be AC2. Because element A needs two electrons to complete its octet, two atoms of element C donate two electrons to element A and get stabilised.

Page No 171:

Question 75:

An element X of atomic number 12 combines with an element Y of atomic number 17 to form a compound XY2. State the nature of chemical bond in XY2 and show how the electron configurations of X and Y change in the formation of this compound.

Answer:

Compound XY2 will have ionic bonds because it is formed by transfer of electrons from atoms of element X to atoms of element â€‹Y.
On forming the compound XY2, each atom of element X that has 12 electrons with an electronic configuration of 2,8,2, donates its two electrons to give X2+ ion that has an electronic configuration of 2,8. Similarly, each atom of Y that has 17 electrons with electronic configuration 2,8,7 completes its octet by gaining one electron to form Y- ion that has an electronic configuration of 2,8,8.

Page No 171:

Question 76:

The electronic configurations of three elements A, B and C are as follows:

A 2, 8, 1
B 2, 8, 7
C 2, 4
(a) Which of these element is a metal?
(b) Which of these elements are non-metals?
(c) Which two elements will combine to form an ionic bond?
(d) Which two elements will combine to form a covalent bond?
(e) Which element will form an anion of valency 1?

Answer:

(a) Element A is a metal because it can donate its one electron to attain noble gas configuration.

(b) Elements B and C are non-metals because B and C can complete their octets either by accepting or sharing the electrons.

(c) Elements A and B will combine to form an ionic bond because element A will transfer its electron to element B to attain noble gas configuration.

(d) Elements B and C will combine to form a covalent bond by sharing four electrons of one atom of element C with four atoms of element B.

(e) Element B will form an anion of valency one because element B contains one less electron to complete its octet. After gaining one electron, it becomes stabilised and forms an anion B-.



Page No 172:

Question 77:

The electronic configurations of four particles A, B, C and D given below:

A 2, 8, 8
B 2, 8, 2
C 2, 6
D 2, 8
Which electronic configuration represents:
(i) magnesium atom?
(ii) oxygen atom?
(iii) sodium ion
(iv) chloride ion?

Answer:

(i) Electronic configuration of particle B represents magnesium atom because its atomic number is 12.

(ii) Electronic configuration of particle C represents oxygen atom because its atomic number is 8.

(iii) Electronic configuration of particle D represents sodium ion. Atomic number of sodium is 11 and it loses one electron to stabilise and forms sodium ion containing 10 electrons.

(iv) Electronic configuration of particle A represents chloride ion. Atomic number of Chlorine is 17 and it gains one electron to stabilise and forms chloride ion containing 18 electrons.

Page No 172:

Question 78:

The atomic number of an element X is 12.
(a) What must an atom of X do to attain the nearest inert gas electron configuration?
(b) Which inert gas is nearest to X?

Answer:

(a) An atom of X must lose its two electrons to attain the nearest inert gas electron configuration.

(b) After losing two electrons, it will have 10 electrons with electronic configuration 2,8, which represents neon gas. So, neon gas is the nearest inert gas to X.

Page No 172:

Question 79:

The atomic number of an element Y is 16.
(a) What must an atom of Y do to achieve the nearest inert gas electron arrangement?
(b) Which inert gas is nearest to Y?

Answer:

(a) An atom of Y  has an electronic configuration of 2,8,6; therefore, it must gain two electrons to achieve the nearest inert gas electron configuration. 

(b) Argon gas is the nearest inert gas to Y. After gaining two electrons, it will have 18 electrons with an electronic configuration of 2,8,8, which represents argon gas.

Page No 172:

Question 80:

You can buy solid air-freshners in shops. Do you think these substance are ionic or covalent? Why?

Answer:

These substances are covalent in nature because bonding between covalent substances are not as strong as ionic substances. So, covalent compounds hold together with weak forces and low melting point. This is why, they vapourise easily and give their odours. However, when ionic compounds comes in contact with air, they cannot give an odour by vapourising.

Page No 172:

Question 81:

Give the formulae of the chlorides of the elements X and Y having atomic numbers of 3 and 6 respectively. Will the properties of the two chlorides be similar or different? Explain your answer.

Answer:

The formula of  the chloride of element X is XCl (or X+Cl-). Element X has an electronic configuration of 2,1. When it reacts with chlorine with electronic configuration 2,8,7, it transfers one electron to chlorine atom to form an ionic chloride compound.

The formula of the chloride of element Y is YCl4. Element Y has an electronic configuration of 2,4. When it reacts with chlorine with electronic configuration 2,8,7, it shares its four electrons with four chlorine atoms.

Properties of the two chlorides will be different because of the difference in the natures of bonding between them. For example, XCl, being ionic in nature, must have high melting and boiling points but YCl4, being covalent in nature, must have low melting and boiling points.



Page No 191:

Question 1:

A zinc ore gave CO2 on treatment with a dilute acid. Identify the ore and write its chemical formula.

Answer:

The zinc ore that gives CO2 on treatment with a dilute acid is calamine.
Its chemical formula is ZnCO3.

Page No 191:

Question 2:

What chemical process is used for obtaining a metal from its oxide?

Answer:

Reduction is the chemical process used for obtaining a metal from its oxide by using reducing agents like carbon, sodium, aluminium or calcium.

Page No 191:

Question 3:

State two ways to prevent the rusting of iron.

Answer:

Rusting of iron can be prevented by the following two ways:

  1. By coating the surface of iron with paint: On coating, air and moisture cannot comes in contact with iron and hence, no rusting takes place.
  2. By applying grease or oil on the surface of iron: This method also prevents air and moisture to come in contact with iron and hence, no rusting takes place.

Page No 191:

Question 4:

What is meant by galvanisation? Why is it done?

Answer:

Galvanisation is a process wherein a thin layer of zinc metal gets deposited on iron objects. This is done to prevent rusting of iron by protecting it to come in contact with air and moisture. Zinc metal, being more reactive, reacts with air to form hard layer of zinc oxide, which prevents air from passing through it.

Page No 191:

Question 5:

Name the metal which is used for galvanising iron.

Answer:

Zinc metal is used for galvanising iron because it is more reactive than iron. It reacts with air to form a hard layer of zinc oxide, which prevents air and moisture to come in contact with iron.

Page No 191:

Question 6:

Explain why, iron sheets are coated with zinc.

Answer:

Iron sheets are coated with zinc because zinc is a reactive metal which readily reacts with oxygen present in air to form a hard layer of zinc oxide. This layer is so hard that it prevents air and moisture from passing through it. Hence, moist air cannot come in contact with iron. This in turn prevents the rusting of iron.

Page No 191:

Question 7:

Why do we apply paint on iron articles?

Answer:

Iron articles are coated with paint to prevent air and moisture from coming in contact with iron and hence, to protect it from rusting.

Page No 191:

Question 8:

Give reason for the following:
Carbonate and sulphide ores are usually converted into oxides during the process of extraction of metals.

Answer:

It is easier to extract metals from their oxide ores by reduction process than their carbonate and sulphide ores. Hence, carbonate and sulphide ores are usually converted into oxides during the process of extraction of metals.

Page No 191:

Question 9:

Name a reducing agent that may be used to obtain manganese from manganese dioxide.

Answer:

Aluminium powder is the reducing agent that may be used to obtain manganese from manganese dioxide.

Page No 191:

Question 10:

Name an alloy of lead and tin.

Answer:

Solder is an alloy of lead and tin. It contains 50 per cent lead and 50 per cent tin.

Page No 191:

Question 11:

Give the composition of an alloy called solder. State its one property and one use.

Answer:

The composition of solder alloy is 50 per cent lead (Pb) and 50 per cent tin (Sn).
It has a very low melting point that is quite less than its constituting metals (Pb and Sn). Thus, solder is widely used in soldering and welding of electrical wires.

Page No 191:

Question 12:

What is an amalgam?

Answer:

Amalgam is an alloy of mercury with one or more metals. Sodium along with liquid mercury is known as sodium amalgam. Amalgam of mercury with tin, silver and zinc is widely used in dental filling.

Page No 191:

Question 13:

How many carats is pure gold? Why is pure gold not suitable for making ornaments?

Answer:

24-carat gold is pure gold. Pure gold is too soft and it is not possible to make ornaments from this soft gold. Thus, 24-carat gold is mixed with two parts of copper or silver for making jewellery. The gold used for jewellery is 22-carat gold.

Page No 191:

Question 14:

Name one method for the refining of metals.

Answer:

Electrolytic refining method is the most significant method used for refining of metals. Several metals like Cu, Zn, Au, Cr, Pb, Sn, Ni and Ag are refined by this process.

Page No 191:

Question 15:

State two conditions for the rusting of iron.

Answer:

Two necessary conditions required for rusting of iron are:
(i) Existence of air (oxygen)
(ii) Existence of water (or dampness)
In the absence of any of the above conditions, rusting of iron will not take place.

Page No 191:

Question 16:

In one method of rust prevention, the iron is not coated with anything. Which is this method?

Answer:

Alloying of iron with chromium and nickel to produce stainless steel (that does not corrode) is one method where iron is not coated with anything.

Page No 191:

Question 17:

Name two alloys of iron. What elements are present in these alloys?

Answer:

Two alloys of iron are steel and stainless steel. Steel consists of iron with 0.1 to 1.5 percent carbon. The elements present in stainless steel are iron, nickel and chromium.

Page No 191:

Question 18:

Give reason for the following:
Silver, gold and platinum are used to make jewellery.

Answer:

Gold, silver and platinum are least reactive metals. They do not react with air, water, acids, alkalis and other gases in the environment. Due to this, their shine lasts for years. Due to bright lustre and resistance towards corrosion, gold, silver and platinum are used to make jewellery.

Page No 191:

Question 19:

Which metal becomes black in the presence of hydrogen sulphide gas in air?

Answer:

Silver metal becomes black in the presence of hydrogen sulphide gas in the air. Silver reacts with hydrogen sulphide gas in the air and forms a thin black layer of silver sulphide (Ag2S) on it. Therefore, silver jewellery or ornaments become dull due to the formation of silver sulphide coating on their surface.

Page No 191:

Question 20:

Name the gas in air which tarnishes silver articles slowly.

Answer:

Hydrogen sulphide gas in the air tarnishes silver articles slowly. Silver reacts with hydrogen sulphide gas in the air and forms a thin black layer of silver sulphide (Ag2S) on it. Therefore, silver jewellery or ornaments become dull due to the formation of silver sulphide coating on their surface.

Page No 191:

Question 21:

Silver metal does not combine easily with oxygen but silver jewellery tarnishes after some time. How?

Answer:

Silver metal does not react with oxygen but reacts with hydrogen sulphide gas in the air and forms a thin black layer of silver sulphide (Ag2S) on it. Therefore, silver jewellery or ornaments tarnish due to the formation of a silver sulphide coating on their surfaces.

Page No 191:

Question 22:

Write the composition of the alloy called bronze. Give two uses of bronze.

Answer:

The composition of bronze is copper (90%) and tin (10%). This alloy of copper is strong and prevents corrosion.
Two uses of bronze are:
(i) It is used in the manufacturing of statues, medals etc.
(ii) It is used to make cooking vessels and propellers of ship.

Page No 191:

Question 23:

Why does a new aluminium vessel lose shine so soon after use?

Answer:

Aluminium is highly reactive. It reacts with moist air and forms a thin layer of aluminium oxide on it. This layer fades away the shine of aluminium. Thus, aluminium vessels lose its shine soon after use.

Page No 191:

Question 24:

Why do gold ornaments look new even after several years of use?

Answer:

Gold is a least reactive metal. It does not react with air, water, acids, alkalis and other gases in the environment. Due to this non-reactive nature, gold metal's shine lasts for years and gold ornaments look new even after several years of use.

Page No 191:

Question 25:

Name two metals which are highly resistant to corrosion.

Answer:

Gold and platinum are highly resistant to corrosion. They are least reactive metals and therefore, they do not react with air, water, acids, alkalis and other gases in the environment.

Page No 191:

Question 26:

Which property of 'solder' alloy makes it suitable for welding electrical wires?

Answer:

Solder is an alloy of lead and tin. Its composition is 50 percent lead (Pb) and 50 percent tin (Sn). The melting point of solder is very low and is even lower than its constituting metals (Pb and Sn). Thus, the low melting point of solder makes it suitable for welding electrical wires.

Page No 191:

Question 27:

Explain why, carbon cannot reduce oxides of sodium or magnesium.

Answer:

Sodium and magnesium are most reactive metals (placed at the top in the reactivity series). They are very stable and cannot be reduced by carbon. They have more attraction to oxygen than carbon. Thus, carbon cannot combine with oxygen and reduce these metal oxides to metals.

Page No 191:

Question 28:

Why are the metals like Na, K, Ca and Mg never found in their free state in nature?

Answer:

Sodium, potassium, calcium and magnesium are most reactive metals (placed at the top in the reactivity series). Due to their high reactivities, they are never found as free elements. They are always found in combined form.

Page No 191:

Question 29:

Name one metal each which is extracted by:
(a) reduction with carbon.
(b) electrolytic reduction.
(c) reduction with aluminium
(d) reduction with heat alone.

Answer:

Let us name one metal in each of the following cases:

(a) Reduction with carbon: Zinc is a moderately reactive metal and thus, its oxide is reduced by carbon.

(b) Electrolytic reduction: Sodium is a highly reactive metal and it cannot be reduced by carbon or aluminium. Thus, it is reduced by electrolysis.

(c) Reduction with aluminium: Manganese is a reactive metal and it cannot be reduced by carbon. Thus, it is reduced by aluminium.

(d) Reduction with heat alone: Mercury is a least reactive metal and thus, it is reduced on heating alone.

Page No 191:

Question 30:

Fill in the following blanks with suitable words:
(a) The corrosion of iron is called ................
(b) ............ and .............. are necessary for the rusting of iron.
(c) The process of depositing a thin layer of zinc on iron articles is called .............
(d) Tiffin boxes are electroplated with .............. but car bumpers are electroplated with ............... to protect them from rusting.
(e) The corrosion of copper produces a .............. coating of basic copper carbonate on its surface.
(f) Brass is an alloy of copper and .........
(g) Bronze is an alloy of copper and .........
(h) The non-metal present in steel is ............
(i) The alloy in which one of the metals is mercury is called an ...............
(j) The electrical conductivity and melting point of an alloy is ........... than that of pure metals.
(k) The rocky material found with ores is called.............

Answer:

(a) The corrosion of iron is called rusting.

(b) Air (oxygen) and water are necessary for the rusting of iron.

(c) The process of depositing a thin layer of zinc on iron articles is called galvanization.

(d) Tiffin boxes are electroplated with tin but car bumpers are electroplated with chromium to protect them from rusting.

(e)
The corrosion of copper produces a green  coating of basic copper carbonate on its surface.

(f) Brass is an alloy of copper and zinc .

(g)
Bronze is an alloy of copper and tin.

(h)
The non-metal present in steel is carbon.

(i)
The alloy in which one of the metals is mercury is called an amalgam

(j)
The electrical conductivity and melting point of an alloy is less than that of pure metals.

(k) The rocky material found with ores is called gangue.



Page No 192:

Question 31:

How is manganese extracted from manganese dioxide, MnO2? Explain with the help of an equation.

Answer:

Manganese is extracted from its oxide (Manganese dioxide or MnO2 ) by using aluminium as the reducing agent. Because aluminium is more reactive than manganese and is placed above it in the reactivity series, aluminium can easily displace manganese from its oxide to liberate the metal.

Manganese dioxide is heated with aluminium powder to produce manganese.

3MnO2 (s) + 4Al (s)  3Mn (l) + 2Al2O3 (s) + Heat

The reaction is highly exothermic and huge amount of heat is liberated. Thus, manganese will be produced in molten (liquid) state.

Page No 192:

Question 32:

What is a thermite reaction? Explain with the help of an equation. State one use of this reaction.

Answer:

A thermite reaction is a reaction in which a metal oxide is reduced by using aluminium as the reducing agent. It is generally exothermic in nature, wherein a large amount of heat is liberated. Due to this heat, the metals produced are in the molten state.
An example of thermite reaction is the reduction of iron (III) oxide by using aluminium powder

Fe2O3 sIron III oxide + 2Al sAluminium powder  2 Fe lIron + Al2O3 sAluminium oxide + Heat

Iron metal is produced when a mixture of iron (III) oxide and aluminium powder is ignited with a magnesium ribbon. The iron, so obtained, is in the molten state due to the large amount of heat liberated in the process. This is used to weld broken pieces of iron.

The above reaction is widely used in thermite welding to join broken pieces of iron.

Page No 192:

Question 33:

Which one of the methods given in column I is applied for the extraction of each of the metals given in column II:

Column I Column II
Electrolytic reduction Aluminium
Reduction with Carbon Zinc
Reduction with Aluminium Sodium
  Iron
  Manganese
  Tin

Answer:

Column I                                    Column II
                               
Electrolytic reduction                  Aluminium, sodium
Reduction with carbon               Zinc, iron, tin
Reduction with aluminium           Manganese

Page No 192:

Question 34:

(a) Give reason why copper is used to make hot water tanks but steel (an alloy of iron) is not.
(b) Explain why, the surface of some metals acquires a dull appearance when exposed to air for a long time.

Answer:

(a) Copper is a less reactive metal compared to iron. It reacts very slowly with water. However, steel has less resistance to corrosion, even though it is an alloy of iron. Thus, copper, being very less reactive than steel, is used to make hot water tanks but steel (an alloy of iron) is not.

(b)
Most of the metals are reactive towards air. They react with oxygen present in the air to form metal oxides. The metal oxide formation appears on the surface of the metals as a thin layer. This thin layer formation of metal oxide on the metallic surface causes the dull appearance of the metals. Thus, the surface of some metals acquires a dull appearance when exposed to air for a long time.

Page No 192:

Question 35:

(a) Why does aluminium not corrode right through?
(b) What is meant by 'anodising'? Why is it done?

Answer:

(a) Aluminium is a highly reactive metal. It reacts easily with oxygen in the air and forms a layer of aluminium oxide on its surface. This layer is very tough and does not allow air to enter the metal for further reaction. This prevents aluminium from further corrosion.  Thus, aluminium articles become unreactive toward air and water and do not corrode easily due to the presence of strong aluminium oxide layer on it. Hence, aluminium is highly resistive to corrosion.

(b) Anodising is a method that involves the formation of tough layer of aluminium oxide on aluminium substances by making it anode in the electrolysis of sulphuric acid. Generally, the formation of aluminium oxide layer on aluminium protects it from corroding further. However, if this oxide layer on aluminium is made thicker (by anodising), it will become a strong coating and will prevent corrosion to the maximum.

Page No 192:

Question 36:

(a) Why is an iron grill painted frequently?
(b) Explain why, through aluminium is more reactive than iron, yet there is less corrosion of aluminium when both are exposed to air.

Answer:

(a) To prevent rusting of iron substances, they are painted frequently. Iron is a reactive metal and it starts corroding in the presence of air (oxygen) and water (or moisture). Iron reacts with oxygen present in the moist air and forms a brown flaky iron oxide (or rust) layer on its surface. This is called rusting. This rusting causes huge damage to iron substances by making them weak. Therefore, an iron grill is painted frequently to prevent corrosion.

(b) Aluminium is more reactive than iron. It reacts easily with oxygen in moist air and forms a layer of aluminium oxide on its surface. This aluminium oxide layer is very strong and does not allow the air to enter the metal for further reaction. This prevents further corrosion of aluminium. On the other hand, the formation of iron oxide layer by iron on reacting with moist air does not prevent the metal. It further allows the reaction between iron and moist air to continue and increases corrosion. Thus, aluminium is a highly resistive metal to corrosion and protects itself against corrosion as compared to iron when exposed to air.

Page No 192:

Question 37:

(a) Name the method by which aluminium metal is extracted.
(b) Give the name and chemical formula of one ore of copper.
(c) How is zinc extracted from its carbonate or (calamine)? Explain with equations.

Answer:

(a) Aluminium is extracted by electrolytic reduction from molten aluminium oxide. 

(b) One of the ores of copper is copper glance. The formula of the ore is Cu2S.

(c) Calamine, zinc carbonate
ZnCO3, can be extracted by calcination. As zinc is moderately reactive and is placed at the middle of the reactivity series, it can be reduced by carbon. The steps involved in the extraction of zinc are:
1. Concentrated carbonate of zinc is converted to its oxide by calcination. In this process, zinc carbonate ore is strongly heated in the absence of air to produce zinc oxide and carbon dioxide gas.

         ZnCO3(s)       Calcination     ZnO(s)    +    CO2(g)


2. Zinc oxide is then reduced by coke (carbon). When zinc oxide is heated with coke, carbon acts as a reducing agent and reduces zinc oxide to zinc.
    ZnO(s)    +   C(s)     reduction      Zn(s)     +      CO(g)                

Page No 192:

Question 38:

(a) Name two metals which occur in nature in free state as well as in combined state.
(b) Name one ore of manganese. Which compound of manganese is present in this ore? Also write its chemical formula.
(c) A zinc ore on heating in air forms sulphur dioxide. Describe briefly any two stages involved in the conversion of this concentrated ore into zinc metal.

Answer:

(a) Copper and silver metals occur in nature in free as well as combined state.

(b) One ore of manganese is pyrolusite. This is an oxide ore. Manganese dioxide is the compound present in pyrolusite. Its chemical formula is
MnO2.

(c) If zinc ore forms sulphur dioxide o
n heating in the presence of air, it should be a sulphide ore, i.e., zinc blende (Zinc sulphide or ZnS).
Two stages involved in the conversion of this concentrated zinc blende to zinc metal are:

1. Roasting: Z
inc blende ore is subjected to roasting, i.e., it is heated in the presence of excess air. This converts zinc sulphide ore to zinc oxide. The reaction involved is:
          2ZnS(s)  +  3O2(g)     Surplus of airRoasting     2ZnO(s) + 2SO2(g)  

2. Reduction: Zinc oxide is then reduced by coke (carbon). When zinc oxide is heated with coke, carbon acts as reducing agent and reduces zinc oxide to zinc metal.
      ZnO(s) + C(s)    reduction    Zn(s) +  CO(g)                           

Page No 192:

Question 39:

How does the method used for extracting a metal from its ore depend on the metal's position in the reactivity series? Explain with examples.

Answer:

Reactivity series depicts the reactivities of metals in decreasing order. It is very helpful in the extraction of metals. The extraction of metals depends on their reactivities.

For extraction purpose, metals in the reactivity series are ordered under three sections:
1. Highly reactive metals
2. Moderately reactive metals
3. Low reactive metals

Depending on the reactivities, different methods are applied for extracting metals from their ores.
Sodium, potassium, calcium, magnesium and aluminium are highly reactive metals and placed at the top of the reactivity series. These metals are extracted by electrolytic reduction of their molten chlorides or oxides because these metals are not reduced by other reducing agents like coke, carbon monoxide etc.

Moderately reactive metals like zinc, iron, lead and copper are placed at the middle of the reactivity series. They are reduced by carbon because carbon is more reactive than these metals and can reduce their metal oxides to metal easily. However, manganese is more reactive than zinc (above zinc in reactivity series); therefore, it cannot be reduced by carbon. It is reduced by a more reactive reducing agent aluminium. Aluminium reduces manganese dioxide to manganese on heating.

Low reactive metals like copper and mercury are placed at the bottom of the reactivity series and are extracted by heating their ores alone. We should note that ores of copper can be extracted by both - reduction with carbon (copper (I) oxide) and heating alone (copper (I) sulphide).
 
Refer to the following diagram for quick glance of extraction of metals based on their reactivities:

Page No 192:

Question 40:

Explain giving one example, how highly reactive metals (which are high up in the reactivity series) are extracted.

Answer:

Sodium, potassium, calcium, magnesium and aluminium are highly reactive metals and are placed at the top of the reactivity series. These metals are extracted by electrolytic reduction of their molten chlorides or oxides because these metals are not reduced by other reducing agents like coke, carbon monoxide etc.
 
Let us take an example of extraction of sodium metal. Sodium metal is extracted by electrolytic reduction of molten sodium chloride. On passing electricity through molten sodium chloride, decomposition reaction occurs and formation of sodium metal and chlorine gas takes place. 

             2NaCl(l)        electrolysis       2Na(s)     +      Cl2(g)

Molten sodium chloride consists of sodium and chloride ions. The reactions that occurs during electrolysis are:

1. Cathode produces electrons to reduce sodium ions to sodium atoms (or sodium metal) by acting as a reducing agent.  Sodium ions are cations; therefore, they get attracted to negatively charged cathode and get deposited there.
Cathode:       2Na+         +       2e-                    2Na                  Sodium ions           Electrons            Sodium atoms             (from molten NaCl)      from cathode      (or sodium metal)    
 
2. Chloride ions, being anions, get attracted to positively charged anode. These chloride ions are oxidised to chlorine gas. Chlorine gas is produced at anode.
 
Anode:         2Cl-         -        2e-                 Cl2              Chloride ions            electrons           Chlorine gas           (from  molten NaCl)      to anode

Page No 192:

Question 41:

Describe with one example, how moderately reactive metals (which are in the middle of reactivity series) are extracted.

Answer:

The oxides of moderately reactive metals like zinc, iron, lead copper, placed in the middle of the reactivity series, are reduced by carbon because carbon is more reactive.

Let us take an example of extraction of zinc from its carbonate ore, i.e., calamine.
The steps involved in the extraction of concentrated calamine ore are:

1. Carbonate of zinc is converted to its oxide by calcination process. In calcination process, zinc carbonate ore is heated
strongly in the absence of air to produce zinc oxide and carbon dioxide gas.
ZnCO3           Absence of airCalcination        ZnO     +  CO2Zinc carbonate                     Zinc Oxide    Carbon dioxide(Calamine ore)


2. Zinc oxide is then reduced by coke (carbon). When zinc oxide is heated with coke, carbon acts as a reducing agent and reduces zinc oxide to zinc metal.
         ZnO    +      C     Reduction        Zn          +     COZinc Oxide          Coke                   Zinc metal     Carbon monoxide gas                       (reducing agent)

Page No 192:

Question 42:

How are the less reactive metals (which are quite low in the reactivity series) extracted? Explain with the help of an example.

Answer:

The ores of less reactive metals like copper and mercury are placed at the bottom of the reactivity series. Metals are extracted from these ores by heating them alone. We should note that ores of copper can be extracted by reduction with carbon (copper (I) oxide) as well as heating alone (copper (I) sulphide).

Let us explain this by taking the example of extraction of mercury from its cinnabar ore (HgS). Cinnabar is a sulphide ore. On heating the ore, mercury can be extracted. It is a sulphide ore; therefore, it is first heated in the presence of surplus air (roasting). This converts mercury (II) sulphide to mercury (II) oxide.
              2HgS         +      3O2     Surplus of airRoasting       2HgO           +       2SO2                                      
Mercury (II) oxide is then heated to around 300o C. On heating, it gets reduced (decomposes) to mercury metal and oxygen gas is released.

         2HgO           Heating 300o CReduction           Hg       +      O2Mercury (II) oxide                        Mercury metal      Oxygen gas

Page No 192:

Question 43:

What is meant by refining of a metal? Name the most widely used method for the refining of impure metals obtained by various reduction processes. Describe this method with the help of a labelled diagram by taking the example of any metal.

Answer:

The metals obtained by different reduction processes are usually found to be with some impurities. The process involved in the removal of these impurities from the reduced metals is called purification or refining of metals. There are several refining methods used to purify metals based on the type of metals.
The most widely used method for the refining of impure metals obtained by various reduction processes is electrolytic refining method. Electrolytic refining is a process of purification by electrolysis.  Several metals like copper, zinc, tin, lead, nickel, chromium, gold and silver are refined by this process.


Let us explain electrolytic refining process by considering an example of copper metal. The electrolytic refining tank consists of anode, cathode and electrolytic solution. In the refining of copper metal, a bulky piece of impure copper is made anode (attached to positive terminal) and a thin layer of pure copper is made cathode (attached to negative terminal). The acidified copper sulphate solution (copper sulphate solution with dilute sulphuric acid) is taken as electrolytic solution or electrolyte.

 
When electricity is passed through the cell, impure copper from the anode (as Cu2+ ions) melts and travels to the electrolyte. The pure copper metal (
Cu2+ ions) from the electrolyte moves to the cathode and gets deposited there as pure copper. The amount of copper ions from the anode going to the electrolyte will be equivalent to the amount of copper ions collected at the cathode. This keeps the concentration of the electrolyte constant and proceeds the further reaction. Copper ions, being positively charged, get attracted to the negatively charged cathode and are collected or deposited there. 
The copper sulphate electrolyte contains
Cu2+ and SO42- ions. The reactions that take place at anode and cathode when electricity passed through the electrolytic cell are:

(i) The impure copper atoms from anode lose two electrons to form copper ions and enter the copper sulphate electrolyte.
Anode:            Cu  -     2e-     Oxidation    Cu2+                            
(ii) The copper ions from the copper sulphate electrolyte move to cathode (negatively charged) by gaining two electrons from cathode and get reduced to copper atoms.
Cathode:           Cu2+ +    2e-     reduction      Cu
 
Thus impure copper atoms, entering copper sulphate electrolyte from anode, get deposited as pure copper atoms at cathode. As the process continues, cathode (pure metal collected) becomes thicker and anode (impure metal) becomes thinner. The metal obtained would be 99.99 percent pure. This pure copper is separated from the cathode later. The impurities are thrown back under the anode as anode mud.​

Page No 192:

Question 44:

(a) Define the terms (i) mineral (ii) ore, and (iii) gangue.
(b) What is meant by the 'concentration of ore'?
(c) Name one ore of copper (other than cuprite). Which compound of copper is present in this ore? Also, write its chemical formula.

Answer:

(a) (i) Minerals: Naturally occurring metals or its compounds are called minerals.
   
(ii) Ore: Ore is a type of mineral from which metal can be extracted profitably and conveniently.
   
(iii) Gangue: The impurities like sand, mud, rocky substances etc. that are present in an ore are called gangue.

(b) An ore usually contains some undesired impurities like sand, mud, rocky substances etc. that are present with the metal. These impurities are called gangue. These impurities are required to be removed before extracting the metal from its ore. Several physical and chemical methods are used to remove these gangue substances from the ore. This process of removing gangue substances from the ore to enhance the concentration (percentage) of metal is called the concentration of the ore.

(c) 
An ore of copper, other than cuprite, is copper glance. Copper glance is a sulphide ore. The compound present in the ore is copper (I) sulphide. The chemical formula of the ore is Cu2S.



Page No 193:

Question 45:

Explain how, a reduction reaction of aluminium can be used for welding cracked machine parts of iron. Write a chemical equation for the reaction involved.

Answer:

When iron (III) oxide and aluminium powder are heated together, aluminium acts as a reducing agent and reduces iron (III) oxide to iron metal. It also gets oxidised to aluminium oxide with the liberation of excess heat.  As excess heat is liberated in the reaction, the product, i.e., iron metal  is formed in molten state.  This reaction is an oxidation-reduction reaction and its chemical equation is:

         Fe2O3(s)  +  2Al(s)   heat       2Fe(l)  +  Al2O3(s)  + Excess heat                                    


This reaction is called as thermite reaction. This reaction is widely used to weld cracked machine parts of iron. This is called thermite welding.

Page No 193:

Question 46:

(a) What is corrosion?
(b) Name any two metals which do not corrode easily.
(c) What is the corrosion of iron known as?
(d) Explain why, aluminium is a highly reactive metal, still it is used to make utensils for cooking.

Answer:

(a) Corrosion is the damage caused to the metal by the chemical reaction of air, water and acids with the surface of the metal. Generally reactive metals corrode easily and non-reactive metals have good resistance to corrosion.

(b) Gold and platinum are two metals that do not corrode easily. These two metal are highly non-reactive; therefore, they have a very high resistance to corrosion.

(c) The corrosion of iron is known as rusting. Iron is a reactive metal and corrodes (rusts) in the presence of air (oxygen) and water (dampness).

(d) Aluminium is a highly reactive metal. It reacts easily with oxygen in the air and forms layer of aluminium oxide on its surface. This aluminium oxide layer is very tough and does not allow the air to enter the metal for further reaction. This prevents further corrosion of aluminium. Therefore, aluminium articles become unreactive towards air and water and do not corrode easily due to the presence of strong aluminium oxide layer over them. This oxide layer on aluminium is made further strong by making it thick by a process called anodising. This prevents corrosion to the maximum. Hence, aluminium becomes highly resistive to corrosion and it is used to make utensils for cooking.

Page No 193:

Question 47:

What is meant by 'rusting of iron'? With the help of labelled diagrams, describe an activity to find out the conditions under which iron rusts.

Answer:

Iron objects, kept in moist air for a long period of time, react with moist air to form a reddish brown crumbling substance called rust (hydrated iron (III) oxide, Fe2O3.X H2O, where X is the number of moles of water that differs). This corrosion of iron metal by reacting with moist air is called the rusting of iron. 
 
Conditions essential for the rusting of iron:
1. Presence of air (oxygen)
2. Presence of water (moisture)
 
Let us do an activity to prove that moist air is necessary for the rusting of iron.

Experiment: Take three test tubes and place a dirt-free iron nail in each of these test tubes (refer to the figure below).
In the first test tube, anhydrous calcium chloride is added and the test tube is closed. In the second test tube, distilled water and a layer of oil is added and the test tube is closed with a cork. In the third test tube, tap water is added. The nail is dipped into water in such a way that a part of it is exposed to the damp air present inside the test tube.



Observation:
(i) The nails in the first and the second test tube do not rust.
ii) The nail present in the third test tube rusts.

Conclusion:
Hence, we can conclude that both air and water are necessary for the rusting of iron.

Page No 193:

Question 48:

(a) What is an alloy? How is an alloy made?
(b) What elements are present in steel? How are the properties of steel different from those of pure iron?
(c) Give the constituents and one use of brass.

Answer:

(a) An alloy is a uniform combination of two or more metals or a metal with a non-metal. It is produced by mixing metals in their molten state in desired quantities. The mixture is then cooled at room temperature. Similarly, an alloy of a metal and a non-metal is prepared by dissolving the non-metal in the molten metal. The mixture is then cooled at room temperature.

(b) Steel is an alloy of iron. It is composed of iron with 0.1 to 1.5 per cent carbon. Iron is not suitable to be used in its pure form because pure iron is very soft and elongates easily when hot. Alloying iron with 0.1 to 1.5 per cent carbon changes the properties of pure iron. This combination is steel that is hard and strong in nature. Steel rusts too but not as easily as iron.

(c) Brass is an alloy of copper and zinc. Composition of brass is copper (80 percent) and zinc (20
percent). Brass is used for making vessels and scientific instruments (e.g.microscope).

Page No 193:

Question 49:

(a) Name two metals which resist corrosion due to the formation of a thin, hard and impervious layer of oxide on their surface.
(b) Name five methods of preventing rusting of iron.
(c) What are the constituents of stainless steel? What are the special properties of stainless steel?

Answer:

(a) Aluminium and zinc are two metals that resist corrosion due to the formation of a thin, hard and impervious layer of oxide on their surface. Aluminium and zinc form tough layers of aluminium oxide and zinc oxide by reacting with air. 

(b) Five methods of preventing rusting of iron are:

  •  By polishing the surface of iron with paint
  •  By applying oil or grease on the surface of iron 
  •  By coating a thin layer of zinc on the iron metal by the process called galvanisation
  •  By coating other metals like tin coating and chromium coating
  •  By alloying with other metals (chromium and nickel) to form stainless steel, which does not corrode
(c) The constituent elements present in stainless steel are iron, nickel and chromium.
Special properties of stainless steel are:
  • It is hard and strong.
  • It does not corrode (does not rust).
These properties make it suitable for making utensils for cooking.

Page No 193:

Question 50:

(a) Name an alloy of copper. State its chemical composition and any one use.
(b) Explain why, when a copper object remains in damp air for a considerable time, a green coating is formed on its surface. What is this process known as?

Answer:

(a) An alloy of copper is bronze. The constituent elements of bronze are copper and tin.
The composition of bronze is 90 percent
copper and 10 percent tin. This alloy of copper is very strong and prevents corrosion.
Bronze is used in the manufacturing of statues, medals, cooking vessels etc.

(b) Copper object, staying in damp air for a long time, reacts with carbon dioxide and moisture (water) in air slowly to form green layer of basic copper carbonate CuCO3.Cu(OH)2 on the outside area of the object.  This process is known as corrosion. Copper is very low in its reactivity; therefore, corrosion of copper also occurs slowly.

Page No 193:

Question 51:

(a) How does the painting of an iron object prevent its rusting?
(b) How does the electrical conductivity of copper alloys, brass and bronze, differ from that of pure copper?
(c) What is meant by 22 carat gold? Name the metals which are usually alloyed with gold to make it harder.

Answer:

(a)  The regular method used to prevent rusting of iron objects is painting the surfaces of iron objects. Rusting takes place in the presence of both air and water. Thus, when a layer of paint is applied to the surface of an iron object, it prevents the contact of air and moisture with the iron surface and avoids the rusting of iron objects.

(b) Alloys of copper, brass (alloy of copper and zinc) and bronze (alloy of copper and tin) are very poor conductors of electricity. However, copper is an excellent conductor of electricity; therefore, it is used in electrical wiring.

(c)
An alloy of gold with copper or silver (Cu and Ag are mixed with gold to make it harder) is called 22 carat gold. The value refers to 22 parts of pure gold alloyed with two parts of either silver or copper.
22 carat gold is used for making jewellery.

Page No 193:

Question 52:

Explain giving equation, what happens when:
(a) ZnCO3 is heated in the absence of air?
(b) a mixture of Cu2O and Cu2S is heated?

Answer:

(a) When zinc carbonate is heated in the absence of air, it is converted to zinc oxide with the evolution of carbon dioxide gas. This is one of the methods used to convert concentrated metal carbonates to metal oxides during extraction of metals. This method is called calcination. The given zinc carbonate is nothing but calamine ore of zinc, which gets converted to zinc oxide on calcination.

       ZnCO3      Calcination     ZnO    +     CO2Zinc carbonate                Zinc oxide    carbon dioxide(Calamine ore)

(b) When a mixture of Cu2O and Cu2S is heated, they react together to give copper metal and sulphur dioxide gas. This is one of the methods used to reduce sulphide ore of less reactive copper to copper metal. Copper glance, 
Cu2S, is roasted in the presence of air to get converted to Cu2O. Cu2O then reacts with non-oxidised Cu2S ore in the absence of air to form copper metal.

        2Cu2O    +    Cu2S   Heated    6Cu   +  SO2      Copper (I)      Copper (I)         Copper     Sulphur        oxide              sulphide          metal     dioxide   

Page No 193:

Question 53:

(a) For the reduction of metal oxide, suggest a reducing agent other than carbon.
(b) Explain why, an aqueous solution of sodium chloride is not used for the electrolytic extraction of sodium metal.

Answer:

(a) Aluminium (Al) can be used as a reducing agent for the reduction of a metal oxide. This is generally used for reducing metal oxides such as manganese dioxide (MnO2). 

(b) If an aqueous solution of sodium chloride is used in electrolysis, sodium metal, formed at cathode, will immediately react with water in the aqueous solution to form sodium hydroxide because sodium is a highly reactive metal. Hence, it is important to use molten sodium chloride and not its aqueous solution in electrolytic extraction.

Page No 193:

Question 54:

How are metals refined by the electrolytic process? Describe the electrolytic refining of copper with the help of a neat labelled diagram.

Answer:

The process of electrolysis is used in electrolytic refining. This is the most widely used method for the refining of impure metals. Several metals like copper, zinc, tin, lead, nickel, chromium, gold, silver are refined by this process.

Electrolytic refining of copper:

An electrolytic refining tank consists of an anode, a cathode and an electrolyte (acidified copper sulphate solution). A thick rod of an impure metal is made the anode and attached to the positive terminal. A thin rod of a pure metal is made the cathode and attached to the negative terminal. When electricity is passed through the cell, impure metal from the anode gets dissolved in the electrolyte. The metal ions from the electrolyte move to the cathode and get deposited there as pure metal. While the soluble impurities dissolve in the electrolyte, the insoluble ones deposit under the anode as anode mud.



 
When electricity is passed through the electrolytic cell, following reactions take place at the anode and cathode:

(i) Impure copper atoms from the anode lose two electrons to form copper ions and enter copper sulphate electrolyte.
Anode:            Cu         -     2e-             Oxidation                   Cu2+(impure metal)    Electrons               (copper ions enters electrolyte)                             (from anode)
(ii) Copper ions from the copper sulphate electrolyte move to the cathode (negatively charged) by gaining two electrons from the cathode. Thus, they get reduced to copper atoms.
Cathode:           Cu2+      +        2e-        reduction              CuCopper ions             Electrons                    Pure copper metal(from electrolyte)    (from cathode)          (deposited at cathode)
As the process continues, the cathode (pure metal collected) progressively becomes thicker and anode (impure metal) keeps getting thinner. The metal obtained is 99.99 percent pure.

Page No 193:

Question 55:

(a) Name the chemical compound which is electrolysed in molten state to obtain aluminium metal. Which gas is evolved during this process?
(b) Name the chemical compound which is electrolysed in molten state to obtain sodium metal. Which gas is produced in this process?
(c) Name the gas produced when calamine ore is calcined.
(d) Name the gas evolved when cinnabar ore is roasted.

Answer:

(a) Aluminium oxide is electrolysed in molten state to obtain aluminium metal. Oxygen gas is evolved during this process.

The equation for this decomposition is:
2Al2O3 (l)    Electrolysis 4Al (l)   +   3O2 (g)Aluminium            Aluminium        Oxygen oxide

(b) â€‹Sodium chloride is electrolysed in molten state to obtain sodium metal. Chlorine gas is evolved during this process.

The equation for this decomposition is:
    2NaCl (l)     Electrolysis 2Na (s)     +     Cl2 (g)Sodium chloride            Sodium          Chlorine

(c) Carbon dioxide (CO2) gas is produced when calamine ore is calcined.
    ZnCO3 (s)       Calcination ZnO (s)     +     CO2 (g)Zinc carbonate              Zinc oxide          Carbon dioxide (Calamine ore)

(d) Sulphur dioxide gas is evolved when cinnabar ore is roasted.
          2HgS(s)   +   3O2 (g)  Roasting 2HgO (s)  +  2SO2 (g)        Mercury (II)    Oxygen               Mercury (II)     Sulphur        sulphide                                         oxide               dioxide     

Page No 193:

Question 56:

(a) Name two metals which are found in nature mainly in the free state (as metallic elements).
(b) Name two metals which are always found in combined state.
(c) What iron compound is present in haematite ore? Also write its chemical formula.

Answer:

(a) Gold (Au) and silver (Ag) are found in nature in free state. This is because they are non-reactive metals.

(b) Sodium (Na) and magnesium (Mg) are always found in combined state. This is because they are highly reactive and their oxides are highly stable.

(c) Haematite ore contains iron (III) oxide. Its chemical formula is Fe2O3.

Page No 193:

Question 57:

(a) What is the difference between a mineral and an ore?
(b) Which metal is extracted from cinnabar ore?
(c) Name one ore of sodium. Name the sodium compound present in this ore and write its chemical formula.
(d) How is sodium metal extracted? Explain with the help of equation of the reaction involved.
(e) Name three other metals which are extracted in a manner similar to sodium.

Answer:

(a) The differences between a mineral and an ore are listed below:

Mineral Ore
1. Naturally occurring metals or their compounds are called minerals. Minerals containing good quantity of metals that can be profitably mined are called ores.
2. Minerals may or may not contain metals in large percentages. Ores always contain metals in large percentages.
3. Minerals may or may nor contain objectionable impurities. Ores do not contain objectionable impurities.
4. All minerals are not ores. All ores are minerals.

(b) Mercury is extracted from cinnabar ore. It is the sulphide ore of mercury and its chemical formula is HgS. The name of the compound present in cinnabar is mercury (II) sulphide. It is a sulphide ore; therefore, it is first heated in the presence of surplus air (roasting). This converts mercury (II) sulphide to mercury (II) oxide.
          2HgS +    3O2    surplus airRoasting    2HgO  + 2SO2                                          
Mercury (II) oxide is then heated again to around 300o C. On heating, it gets reduced to mercury.
            2HgO    300o CHeating     2Hg + O2

(c) The ore of sodium is rock salt. The sodium compound present in this ore is sodium chloride. The chemical formula of the ore is NaCl.

(d) Sodium is extracted by electrolytic reduction of molten sodium chloride. On passing electricity through molten sodium chloride, decomposition occurs and sodium metal and chlorine gas are formed. 
             2NaCl(l)      Electrolysis    2Na(s)   +  Cl2(g)
Molten sodium chloride contains sodium and chloride ions. The reactions that occur during electrolysis are:

1. The cathode produces electrons to reduce sodium ions to sodium atoms (or sodium metal) by acting as reducing agent. Sodium ions are cations; therefore, they are attracted to the negatively-charged cathode and get deposited there.
Cathode:           2Na+ + 2e-     2Na 

2. Chloride ions are anions; therefore they are attracted to the positively-charged anode. These chloride ions are oxidised to chlorine gas. Chlorine gas is produced at the anode.
Anode:              2Cl- -  2e-      Cl2        
 
(e) Potassium, calcium and aluminium are highly reactive metals like sodium and are also placed on the top of the reactivity series. These metals are not reduced by other reducing agents such as coke, carbon monoxide etc.​ Like sodium, these metals are also extracted by electrolytic reduction of their molten chlorides or oxides. 

Page No 193:

Question 58:

(a) Name the metal which is extracted from haematite ore.
(b) Name one ore of aluminium. Name the aluminium compound present in this ore and write its chemical formula.
(c) How is aluminium metal extracted? Explain with the help of an equation.
(d) Name the electrode at which aluminium metal is produced.
(e) Which gas is produced during the extraction of aluminium? At which electrode is this gas produced?

Answer:

(a) The metal extracted from hematite ore is iron. Hematite ore contains iron (III) oxide (Fe2O3). This iron oxide is extracted by heating it with aluminium. Aluminium acts as a reducing agent and reduces iron (III) oxide to iron.

(b) Bauxite is an ore of aluminium. The aluminium compound present in this ore is aluminium oxide. Its chemical formula is Al2O3.2H2O.

(c) ​ Aluminium is extracted by electrolytic reduction of its molten oxide. Aluminium is a highly reactive metal and it is placed at the top of the reactivity series. This cannot be reduced by other reducing agents such as coke, carbon monoxide etc.

On passing electricity through molten aluminium oxide, decomposition reaction occurs and formation of aluminium metal and oxygen gas takes place. 

             2Al2O3(l)    Electrolysis    4Al(s)  + 3O2(g)

Molten aluminium oxide consists of aluminium and oxide ions. The reactions that occur during electrolysis are:

1. Cathode produces electrons to reduce aluminium ions to aluminium atoms (or
aluminium metal) by acting as reducing agent. Aluminium ions are cations; therefore, they get attracted to negatively charged cathode and get deposited there.
Cathode:              Al3+ +   3e-      Al    

2. Oxide ions are anions; therefore, they get attracted to positively charged anode. These oxide ions are oxidised to oxygen gas. Oxygen gas is produced at anode.
Anode:             2O2- -  4e-    O2        

(d) Aluminium metal is produced at cathode. During electrolysis, cathode produces electrons to reduce aluminium ions into aluminium atoms (or 
aluminium metal) by acting as reducing agent.  Aluminium ions are cations; therefore, they get attracted to negatively charged cathode and get deposited there.
Cathode:              Al3+  +  3e-       Al    

(e) Oxygen gas is produced during the extraction of aluminium. This oxygen gas is produced at anode. During electrolysis, aluminium atoms are deposited at cathode, whereas oxide ions, being anions, get attracted to positively charged anode. These oxide ions are oxidised to oxygen gas. Oxygen gas is produced at anode.
Anode:             2O2--  4e-     O2       



Page No 194:

Question 59:

(a) Which metal is extracted from bauxite ore?
(b) Give the name of one ore of iron. Which iron compound is present in this ore? Write its chemical formula.
(c) Describe the extraction of zinc metal from its sulphide ore (zinc blende). Write equations of the reactions involved.
(d) Explain why, the galvanised iron article is protected against rusting even if the zinc layer is broken.
(e) Name a common metal which is highly resistant to corrosion.

Answer:

(a) Aluminium metal is extracted from bauxite ore. The aluminium compound present in this ore is aluminium oxide. The chemical formula of the ore is Al2O3.2H2O.

(b) Hematite is an ore of iron. Iron compound present in hematite ore is iron (III) oxide. The formula of the ore is Fe2O3

(c) Zinc blende (ZnS) is the sulphide ore of zinc. The extraction of zinc from zinc blende involves two steps. First, the concentrated zinc blende is converted to its oxide (by roasting). After this, the oxide is reduced to zinc metal. The reactions for the two processes are: 

1. Roasting: Zinc blende is subjected to roasting, i.e. it is heated in the presence of excess air. This converts zinc sulphide ore to zinc oxide.
The reaction involved is:
           2ZnS + 3O2   Surplus airRoasting   ​2ZnO +2SO2

2. Reduction: Zinc oxide is then reduced by coke (carbon). When zinc oxide is heated with coke, carbon acts as a reducing agent and reduces zinc oxide to zinc metal.
        ZnO +C  Reduction   Zn + CO  

(d) The galvanised iron-zinc oxide layer coating on iron objects prevents zinc and
iron under it​ from corrosion. This happens because zinc is more reactive than iron. If the zinc layer is broken,​ zinc, being more reactive, gets oxidised before iron and prevents its corrosion.

(e) Aluminium is a common metal that is highly resistant to corrosion because it is a highly reactive metal. It reacts easily with oxygen in air and forms a layer of aluminium oxide on its surface. This aluminium oxide layer is very tough and does not allow air to enter the metal for further reaction, thus preventing further corrosion of aluminium.

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Question 60:

(a) Name the metal which is extracted from the ore called 'rock salt'.
(b) Name two ores of zinc. Write the names of the chemical compounds present in them and give their chemical formulae.
(c) Explain how, mercury is extracted from its sulphide ore (cinnabar). Give equations of the reactions involved.
(d) In the electrolytic refining of a metal M, what would you take as anode, cathode and electrolyte?
(e) Name any five metals which are purified by electrolytic refining method.

Answer:

(a) The metal extracted from the ore rock salt is sodium. The sodium compound present in this ore is sodium chloride. The chemical formula of the ore is NaCl.

(b) The two ores of zinc are calamine ore and zinc blende. The chemical compound present in calamine is zinc carbonate (carbonate ore) and its
chemical formula is ZnCO3. The chemical compound present in zinc blende is zinc sulphide (sulphide ore) and its chemical formula is ZnS .

(c) Cinnabar is a sulphide ore of mercury. The name of the compound present in the cinnabar ore is mercury (II) sulphide. Its chemical formula is HgS.  Mercury is a less reactive metal and it can be extracted from its sulphide ore cinnabar by heating it in air alone.

1. It is first heated in the presence of surplus air (roasting process). This converts mercury (II) sulphide to mercury (II) oxide.
           2HgS + 3O2   Surplus airRoasting 2HgO + 2SO2

2. Mercury (II) oxide, so formed, is heated to around 300oC  and it gets reduced to mercury metal.
          2HgO   300oCHeating  2Hg+O2

(d) In electrolytic refining of a metal, the electrolytic refining tank consists of:

Anode: A bulky piece of impure metal (attached to positive terminal)
Cathode: A thin layer of pure metal (attached to negative terminal)
Electrolyte: Salt solution of the metal, which is being refined, is taken as the electrolytic solution or electrolyte. 

When electricity is passed through the cell, impure metal from the anode melts and goes into the electrolyte. The pure metal  from the electrolyte moves to the cathode and gets deposited there as pure metal. The impurities that are soluble dissolve in electrolyte and insoluble
impurities deposit under anode as anode mud.

(e)  Five metals that can be purified by electrolytic refining method are copper, zinc, chromium, lead and nickel.

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Question 61:

(a) Which metal is extracted from calamine ore?
(b) Name one ore of mercury. Which mercury compound is present in this ore? Write its chemical formula.
(c) How is copper extracted from its sulphide ore (copper glance), Cu2S? Explain with equations of the reactions involved.
(d) What is an alloy? Give two examples of alloys.
(e) How are the properties of an alloy different from those of the constitutent elements?

Answer:

(a) Zinc metal is extracted from calamine ore. Calamine ore is a carbonate ore. The compound present in it is zinc carbonate. Its formula is ZnCO3

(b) The ore of mercury is cinnabar. Cinnabar is a sulphide ore. The compound present in it is mercury (II) sulphide. Its chemical formula is HgS.  

(c) The sulphide ore of copper is copper glance. The compound present in the ore is copper (I) sulphide. Its formula is Cu2S. Copper is a less reactive metal; therefore, it is placed at the bottom in the reactivity series. Copper is extracted by heating its sulphide ore alone.

Two steps involved in the extraction of copper metal from concentrated copper glance ore are:

1. Roasting: Copper glance, being a sulphide ore, is heated in the presence of air. Copper (I) sulphide is converted to copper (I) oxide.
           2Cu2S(s)+ 3O2(g)   Surplus airRoasting  2Cu2O(s) +  2SO2(g)

2. Copper (I) oxide, so 
formed, reacts with non-oxidised copper (I) sulphide ore in the absence of air to form copper metal. When the mixture of Cu2O and  Cu2S is heated, they react together to give copper metal and sulphur dioxide gas.
        2Cu2O(s) + Cu2S(s)   Heat    6Cu(s)+ SO2(g)

(d) A uniform combination of two or more metals or a metal with a non-metal is called an alloy. Alloys are produced by mixing metals in their molten state in desired quantities and allowing the solution to cool at the room temperature. Similarly, an alloy of a metal and non-metal is prepared by dissolving the non-metal in the molten metal and allowing the solution to cool at the room temperature.
Examples: Steel is an alloy of iron. The composition of steel is iron with 0.1 to 1.5 % carbon.
                Brass is an alloy of copper and zinc. Its composition is 80 percent
copper and 20 percent zinc. 

(e) The properties of alloys are different from those of the constituent elements :

1. Alloys have higher strength than their constituting metals. Example: Steel is an alloy of iron with carbon and it has higher strength than iron.

2. Alloys are harder than 
their constituting metals. Example: Magnalium is an alloy of aluminium and magnesium and it is harder than aluminium.

3. Alloys have high resistance to corrosion
than their constituting metals.​ Example: Stainless steel is an alloy of iron, nickel and chromium, and it does not corrode at all, whereas iron rusts easily.

4. Alloys have lesser melting points 
than their constituting metals.​ â€‹Example: Solder an alloy of tin and lead and it has a very low melting point, which is less than its constituting metals

5. Alloys have low electrical conduction 
than their constituting metals.​ â€‹Example: Copper is an excellent conductor of electricity but brass (alloy of copper and zinc) and bronze (alloy of copper and tin) are poor conductors of electricity.

Page No 194:

Question 62:

An ore of manganese metal is:
(a) bauxite
(b) haematite
(c) cuprite
(d) pyrolusite

Answer:

(d) Pyrolusite
​Pyrolusite is an ore of manganese and the compound is manganese dioxide (MnO2).

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Question 63:

Which of the following is an iron ore?
(a) cinnabar
(b) calamine
(c) haematite
(d) rock salt

Answer:

(c) Haematite.
Haematite is an iron ore and the compound is iron (III) oxide (Fe2O3
).

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Question 64:

The metal which can be extracted from the bauxite ore is:
(a) Na
(b) Mn
(c) Al
(d) Hg

Answer:

(c) Al
Aluminium can be extracted from bauxite. Bauxite is an ore of aluminium and the compound is aluminium oxide (Al2O3.2H2O).

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Question 65:

The two metals which can be extracted just by heating their sulphides in air are:
(a) sodium and copper
(b) copper and aluminium
(c) potassium and zinc
(d) mercury and copper

Answer:

(d) mercury and copper
Both mercury and copper are less reactive metals and are placed low in the reactivity series. Hence, they can be extracted by heating their sulphides in air and further reducing their oxides by heating.

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Question 66:

A common metal which is highly resistant to corrosion is:
(a) iron
(b) copper
(c) aluminium
(d) magnesium

Answer:

(c) aluminium
Aluminium is resistant to corrosion due to the formation of a thin oxide layer (aluminium oxide or Al2O3). This prevents further corrosion by creating a strong protective layer over the surface of the metal.

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Question 67:

An important ore of zinc metal is:
(a) calamine
(b) cuprite
(c) pyrolusite
(d) haematite

Answer:

(a) calamine
Calamine is an ore of zinc and the compound is zinc carbonate (ZnCO3).

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Question 68:

The major ore of aluminium is known as:
(a) cinnabar
(b) calamine
(c) bauxite
(d) pyrolusite

Answer:

(c) bauxite
Bauxite is one of the major ores of aluminium. Its compound is aluminium oxide (Al2O3.2H2O
).

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Question 69:

The two metals which are extracted by means of electrolytic reduction of their molten salts are:
(a) magnesium and manganese
(b) iron and aluminium
(c) zinc and magnesium
(d) magnesium and aluminium

Answer:

(d) magnesium and aluminium
This is because magnesium and aluminium are highly reactive metals and are placed high in the reactivity series. This is also due to the fact that their oxides are very stable and cannot be reduced easily.

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Question 70:

In stainless steel alloy, iron metal is mixed with:
(a) Cu and Cr
(b) Cr and Ni
(c) Cr and Sn
(d) Cu and Ni

Answer:

(b) Cr and Ni
Stainless steel is an alloy of iron, chromium and nickel. It is extremely strong and tough and does not corrode at all.

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Question 71:

If copper is kept exposed to damp air for a considerable time, it gets a green coating on its surface. This is due to the formation of:
(a) hydrated copper sulphate
(b) copper oxide
(c) basic copper carbonate
(d) copper nitrate

Answer:

(c) basic copper carbonate
Copper reacts slowly with water and carbon dioxide in the atmosphere to form copper carbonate. This gives a green colour coating to its surface.

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Question 72:

Which of the following alloys contains mercury as one of the constituents?
(a) stainless steel
(b) solder
(c) duralumin
(d) zinc amalgam

Answer:

(d) zinc amalgam
Generally, alloys containing mercury as one of its constituents are known as amalgam. Zinc amalgam is an alloy of zinc and liquid mercury.

Page No 194:

Question 73:

Which of the following is an ore of mercury metal?
(a) rock salt
(b) cinnabar
(c) calamine
(d) haematite

Answer:

(b) cinnabar
Cinnabar is an important ore of mercury. Its compound is mercury (II) sulphide (HgS).

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Question 74:

Calamine ore can be used to extract one of the following metals. This metal is:
(a) copper
(b) mercury
(c) aluminium
(d) zinc

Answer:

(d) Zinc.
Calamine is an ore of zinc and its compound is zinc carbonate (ZnCO3).



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Question 75:

Which of the following pair of metals exists in their native state in nature?
(a) Ag and Hg
(b) Ag and Zn
(c) Au and Hg
(d) Au and Ag

Answer:

(d) Au and Ag
Gold (Au) and silver (Ag) are highly non-reactive. Hence, they are available in native state in nature and do not react with oxygen or any other element in the atmosphere.

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Question 76:

Which of the following reactants are used to carry out the thermite reaction required for welding the broken railway tracks?
(a) Al2O3 + Fe
(b) MnO2 + Al
(c) Fe2O3 + Al
(d) Cu2O + Fe

Answer:

(c) Fe2O3 + Al
In this thermite reaction, iron (III) oxide and aluminium powder are combined and heated. Iron oxide gets reduced to iron by the action of reducing agent 
aluminium and produces excess heat. The molten iron produced is poured 
in the middle of the broken pieces of irons that are fixed mutually.

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Question 77:

Which of the following alloys contains a non-metal as one of the constituents?
(a) brass
(b) amalgam
(c) steel
(d) bronze

Answer:

(c) Steel
Steel is an alloy that contains a non-metal carbon as one of its constituents. It consists of iron with carbon (0.1 to 1.5 %).

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Question 78:

During the refining of an impure metal by electrolysis, the pure metal is a deposited:
(a) at cathode
(b) on the walls of electrolytic tank
(c) at anode
(d) at the bottom of electrolytic tank

Answer:

(a) at cathode
In electrolytic refining, pure metal is always deposited at the cathode. The impure metal is made anode (positive terminal) and a thin rod of pure metal is made cathode (negative terminal). The salt of the metal  is made electrolyte in the cell. When electricity passed through the cell, the impure metal dissolves in the electrolytic solution. The pure metal from the electrolytic solutions moves and gets deposited at the cathode.

Page No 195:

Question 79:

Which of the following metals can be obtained from haematite ore?
(a) copper
(b) sodium
(c) zinc
(d) iron

Answer:

(d) Iron
Iron can be obtained from hematite ore. Hematite ore contains the compound iron (III) oxide (Fe2O3).

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Question 80:

Brass is an alloy of:
(a) Cu and Sn
(b) Cu and Pb
(c) Pb and Sn
(d) Zn and Cu

Answer:

(d) Zn and Cu
Brass is an alloy of copper and zinc. It comprises of 80 percent copper and 20 percent zinc.
 
.

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Question 81:

The metal which is always present in an amalgam is:
(a) iron
(b) aluminium
(c) mercury
(d) magnesium

Answer:

(c) mercury
An amalgam is an alloy of mercury with one or more metals. The combination of sodium with liquid mercury is called sodium amalgam. Amalgam of mercury with gold, silver or zinc is widely used
for dental fillings.

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Question 82:

Manganese metal is extracted from manganese dioxide by a reduction process by making use of:
(a) carbon
(b) hydrogen
(c) electrolysis
(d) aluminium

Answer:

(d) aluminium
There are some metals that cannot be satisfactorily reduced by carbon. In such cases, aluminium is used as reducing agent, e.g., manganese.

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Question 83:

The metal which can be extracted simply by heating the cinnabar ore in air is:
(a) Zn
(b) Cu
(c) Al
(d) Hg

Answer:

(d) Hg
Mercury is a low reactive metal. Thus, it can be extracted from its ore cinnabar, HgS, by heating it in air.

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Question 84:

During galvanisation, iron metal is given a thin coating of one of the following metals. This metal is:
(a) chromium
(b) tin
(c) zinc
(d) copper

Answer:

(c) zinc
Galvanisation is a process that involves dipping of iron objects into melted zinc. This gives a thin coating of zinc layer on the iron object that protects it from corrosion. Zinc, being a highly reactive metal, forms a thin layer of zinc oxide, 
which prevents iron from getting exposed to the atmospheric air.

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Question 85:

Which of the following metals are extracted by the electrolysis of their molten chlorides?
(a) Na and Hg
(b) Hg and Mg
(c) Na and Mg
(d) Cu and Fe

Answer:

(c) Na and Mg
Sodium (Na) and magnesium (Mg) are highly reactive metals. They are not easily reduced by carbon and aluminium. Thus, they are extracted by electrolytic reduction or electrolysis of their molten chlorides (sodium chloride, NaCl, and magnesium chloride, MgCl2).

Page No 195:

Question 86:

Rock salt is an ore of one of the following metals. This metal is:
(a) Mn
(b) Na
(c) Fe
(d) Cu

Answer:

(b) Na
Rock salt is a chloride ore of sodium. The name of the compound in the ore is sodium chloride with chemical formula NaCl.

Page No 195:

Question 87:

The articles made of silver metal become dark on prolonged exposure to air. This is due to the formation of a layer of its:
(a) oxide
(b) hydride
(c) sulphide
(d) carbonate

Answer:

(c) sulphide
Silver reacts with hydrogen sulphide gas in the air and forms a think black layer of silver sulphide (Ag2S) on it. Silver jewellery or ornaments become dull because of the formation of silver sulphide coating on their surfaces.

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Question 88:

A sulphide ore is converted into metal oxide by the process of:
(a) carbonation
(b) roasting
(c) calcination
(d) anodising

Answer:

(b) roasting
Sulphide ores are transformed into oxides by the process of roasting. In this process, concentrated ore is heated in the presence of surplus air. Example: Zinc blende, ZnS, a sulphide ore of zinc, is roasted in the presence of air. On roasting​, zinc sulphide gets transformed into zinc oxide.

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Question 89:

The metal which can be extracted from pyrolusite ore is:
(a) mercury
(b) manganese
(c) aluminium
(d) magnesium

Answer:

(b) manganese
Manganese is extracted from its oxide ore, pyrolusite. The name of the compound in the ore is manganese dioxide having chemical formula MnO2.

Page No 195:

Question 90:

Calamine ore can be converted into zinc oxide by the process of:
(a) dehydration
(b) roasting
(c) calcination
(d) sulphonation

Answer:

(c) calcination
Calamine (ZnCO3, zinc carbonate) is a carbonate ore of zinc. Carbonate ores are converted to metal oxides by calcination (heating in the absence of air). When calamine ore (zinc carbonate) is heated in the absence of air, it is converted into zinc oxide.

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Question 91:

Zinc blende ore can be converted into zinc oxide by the process of:
(a) roasting
(b) hydrogenation
(c) chlorination
(d) calcination

Answer:

(a) roasting
Zinc blende (zinc sulphide or
ZnS) is a sulphide ore of zinc. Sulphide ores are converted to metal oxides by roasting (heating in the presence of air). When zinc blende is heated in the presence of surplus air, it is converted into zinc oxide. 

Page No 195:

Question 92:

An element A which is a part of common salt and kept under kerosene reacts with another element B of atomic number 17 to give a product C. When an aqueous solution of product C is electrolysed then a compound D is formed and two gases are liberated.
(a) What are A and B?
(b) Identify C and D.
(c) What will be the action of C on litmus solution? Why?
(d) State whether element B is a solid, liquid or gas at room temperature.
(e) Write formula of the compound formed when element B reacts with an element E having atomic number 5.

Answer:

(a) Element A is sodium and element B is chlorine.
Sodium is kept under kerosene because it is highly reactive. Element with atomic number 17 is chlorine. Thus, element A is sodium and element B is chlorine.
 
(b) When element A, sodium, reacts with element B, chlorine, it forms a product C, sodium chloride, NaCl.
            2Na + Cl2 2NaCl
When aqueous solution of sodium chloride is electrolysed, compound D, sodium hydroxide (NaOH), is formed along with the evolution of hydrogen and chlorine gases.

         NaCl(aq) electrolysis NaOH(s) + H2(g) + Cl2(g)

(c) Compound C is sodium chloride. It has no action on litmus solution. Because sodium chloride is a salt, it is neutral in nature and does not show any action with litmus solution.
 
(d) Element B is chlorine. It is in gaseous state at room temperature.
 
(e) The element E with atomic number 5 is boron. When element B, chlorine, reacts with element E, boron, it forms a compound boron trichloride. The formula of the compound formed is BCl3.
      2B+3Cl2 2BCl3

Page No 195:

Question 93:

A metal which exists as a liquid at room temperature is obtained by heating its sulphide ore in the presence of air.
(a) Name the metal and write its chemical symbol.
(b) Write the name and formula of the sulphide ore.
(c) Give the equations of chemical reactions involved in the production of metal from its sulphide ore.
(d) Name a common device in which this metal is used.
(e) Can this metal displace copper from copper sulphate solution? Why?

Answer:

(a) The metal is mercury and its chemical symbol is Hg.

(b) Sulphide ore of mercury is cinnabar and its chemical formula is HgS (Mercury (II) sulphide).

(c) Mercury is a less reactive metal. Its ore cinnabar, HgS, is a sulphide ore. On heating the sulphide ore, mercury can be extracted. It is a sulphide ore; therefore, it is first heated with air (roasting process). This converts mercury (II) sulphide to mercury (II) oxide.

2HgS +3O2  2HgO + 2SO2
Mercury (II) oxide, thus
formed, is heated to around 300oC and it gets reduced to mercury.
         2HgO  2Hg  +   O2
(d) A common device wherein mercury is used is thermometer.

(e) No, mercury cannot displace copper from its salt solution. Mercury is less reactive than copper and is placed below it in the reactivity series.



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Question 94:

No chemical reaction takes place when granules of a rusty-brown solid A are mixed with the powder of another solid B. However, when the mixture is heated, a reaction takes place between its components. One of the products C is a metal and settles down in the molten state while the other product D floats over it. It was observed that the reaction is highly exothermic.
(a) What could the solids A and B be?
(b) What are the products C and D most likely to be?
(c) Write the chemical equation for the reaction between A and B leading to the formation of C and D. Mention the physical states of all the reactants and products in this equation and indicate the heat change which takes place.
(d) What is the special name of such a reaction? State one use of such a reaction.
(e) Name any two types of chemical reactions under which the above reaction can be classified.

Answer:

(a) The rusty-brown solid A is iron (III) oxide and another solid B is aluminium. Mixing of iron granules with aluminium powder will have no reaction. 

(b) The product are likely to be C (iron) and D (aluminium oxide). Once the mixture of iron (III) oxide granules and aluminium powder is heated, aluminium acts as reducing agent. Aluminium reduces iron (III) oxide to iron and aluminium powder oxidises to aluminium oxide with the liberation of excess heat.

(c)
        Fe2O3(s) + 2Al(s)   heat  2Fe(l) + Al2O3 (s)+Heat                                   
When A (iron (III) oxide) and B (aluminium powder) are heated, iron and aluminium oxide get produced with the liberation of excess heat. Thus, the product C is iron and product D is aluminium oxide. As excess heat is liberated in the reaction, the product C, i.e., iron metal, is in the molten state. 

(d) The special name given to this reaction is thermite reaction. This reaction is widely used to weld broken hard substances like iron in railway tracks. This process of welding is called thermite welding.

(e) The above said reaction can be classified into displacement reaction and oxidation-reduction reaction. Aluminium, being more reactive than iron, displaces iron in iron (III) oxide, undergoing displacement reaction. Aluminium also acts as a reducing agent and reduces iron (III) oxide to iron metal and gets oxidised to aluminium oxide, undergoing oxidation-reduction reaction.

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Question 95:

In an electrolytic tank, aluminium metal is being extracted by the electrolysis of molten aluminium oxide using carbon electrodes. It is observed that one of the carbon electrodes is gradually burnt away and has to be replaced.
(a) Which carbon electrode (cathode or anode) is burnt away?
(b) Why is this carbon electrode burnt away?

Answer:

(a)The carbon electrode acting as anode (positively charged) is burnt away.

(b) In the electrolysis of molten aluminium oxide using carbon electrodes, the pure metal is formed at the cathode (negatively-charged electrode) and oxygen is formed at the anode (positively-charged electrode). This oxygen reacts with positively-charged carbon electrode and produces carbon dioxide gas. Due to this reaction between oxygen and carbon electrode (anode), the carbon electrode burns away gradually and has to be replaced.

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Question 96:

A metal X which is resistant to corrosion is produced by the electrolysis of its molten oxide whereas another metal Y which is also resistant to corrosion is produced by the reduction of its oxide with carbon. Metal X can be used in powder form in thermite welding whereas metal Y is used in making cathodes of ordinary dry cells.
(a) Name the metals X and Y.
(b) Which of the two metals is more reactive : X or Y?
(c) Name one ore or metal X. Also write its chemical formula.
(d) Name one ore of metal Y. Also write its chemical formula.
(e) Name one alloy of metal X and one alloy of metal Y.

Answer:

(a) Metal X is aluminium and metal Y is zinc.
Metal X or aluminium is resistant to corrosion and is produced by electrolysis of molten aluminium oxide. Aluminium is used in thermite welding.
Metal Y or zinc is also resistant to corrosion and is produced by reduction of zinc oxide with carbon (reducing agent). Zinc is widely used in making cathodes of ordinary dry cells.

(b) X (aluminium) is more reactive than Y (zinc). Aluminium is placed above zinc in the reactivity series.

(c) One ore of metal X (aluminium) is bauxite ore. The chemical formula of bauxite ore is Al2O3.2H2O.

(d) One ore of metal Y (zinc) is calamine ore. The chemical formula of calamine ore is ZnCO3.

(e) One alloy of metal X (aluminium) is magnalium, which consists of aluminium and magnesium. One alloy of metal Y (zinc) is brass, which consists of zinc and copper.

Page No 196:

Question 97:

When an object made of metal A is kept in air for a considerable time, it loses its shine and becomes almost black due to the formation of a layer of substance B. When an object made of another metal C is kept in damp air for a considerable time, it gets covered with a green layer of substance D. Metal A is the best conductor of electricity whereas metal C is the next best conductor of electricity.
(a) What is metal A?
(b) What is metal C?
(c) Name the substance B.
(d) Name the substance D.
(e) What type of chemical can be used to remove the green layer from metal C and clean it? Why?

Answer:

(a) Metal A is silver. Silver becomes black in the presence of hydrogen sulphide gas in air. Silver reacts with hydrogen sulphide gas in air and forms a thin black layer of silver sulphide (Ag2S) on it. Therefore, silver jewellery or ornaments become dull due to the formation of silver sulphide coating on their surfaces. Silver is the best conductor of electricity.

(b) Metal C is copper. When copper is kept in moist air, it
slowly reacts with carbon dioxide and moisture (water) in the air to form a green layer of basic copper carbonate (CuCO3.Cu(OH)2). This basic copper carbonate is a combination of copper carbonate and copper hydroxide. After silver, copper is the next best conductor of electricity.

(c) Substance B is silver sulphide
(Ag2S) .

(d) Substance D is 
basic copper carbonate (CuCO3.Cu(OH)2).


(e) Dilute acid solution can be used to remove the green layer on copper. This acid solution dissolves the green layer of basic copper carbonate present on corroded copper. This helps copper to regain its shine.

Page No 196:

Question 98:

Four metals P, Q, R and S are all obtained by the reduction of their oxides with carbon. Metal P is used to form a thin layer over the sheets of metal S to prevent its corrosion. Metal Q is used for electroplating tiffin boxes made of metal S whereas metal R is used in making car batteries. Metals Q and R form an alloy called solder. What are metals P, Q, R and S? How have you arrived at this conclusion?

Answer:

Metal P is Zn (zinc), Q is Sn (tin), R is Pb (lead) and S is Fe (iron).

We arrive to the following conclusion:

Metal P (zinc) is used to form a thin layer on metal S (iron). The process of thin coating of zinc layer on iron objects is called galvanisation. This thin coating of zinc layer on iron objects protects it from corrosion (zinc is a highly reactive metal. It does not rust even in moisture because it forms a thin coat of zinc oxide on reacting with air). 

Metal Q (tin) is used for electroplating tiffin boxes made of metal S (iron). This tin coating prevents the corrosion of iron. However, tin coating provides
only outside protection.

Metal R (lead) is used in making car batteries. Metal Q (tin) and metal R (lead) are combined together to form an alloy called solder.

Page No 196:

Question 99:

A black metal oxide XO2 is used as a catalyst in the preparation of oxygen gas from potassium chlorate. The oxide XO2 is also used in ordinary dry cells. The metal oxide XO2 cannot be reduced satisfactorily with carbon to form metal X.
(a) Name the metal X.
(b) Name the metal oxide XO2.
(c) Which reducing agent can be used to reduce XO2 to obtain metal X?
(d) Name another metal which can also be extracted by the reduction of its oxide with the above reducing agent.

Answer:

(a) Metal X is manganese. 
Manganese dioxide, MnO2 (black in colour), is used as a catalyst in the preparation of oxygen gas from potassium chlorate. Manganese dioxide is also used in ordinary dry cells. Manganese is a reactive metal; therefore, manganese dioxide cannot be reduced satisfactorily with carbon to form manganese metal. 

(b)The metal oxide is manganese dioxide,
MnO2.

(c) Manganese dioxide, MnO2, cannot be reduced satisfactorily with carbon to form manganese metal. Manganese is comparatively more reactive; therefore, aluminium is used in place of carbon as a reducing agent.

(d) Iron can also be extracted when aluminium is used as a reducing agent. This reaction is called thermite reaction. In this reaction, iron (III) oxide is reduced by with aluminium powder to give iron and aluminium oxide. This reaction is used for welding broken railway tracks.

Page No 196:

Question 100:

Metals X and Y can be recovered from the anode mud left behind after the electrolytic refining of copper metal. The coins made of metal X look new even after several years of use but the coins made of metal Y lose their shine gradually and get blackened soon. When metal X is alloyed with a small amount of metal Y, it becomes hard and hence suitable for making ornaments. What are metals X and Y? Also state the colour of metal X.

Answer:

Metals X and Y are gold and silver, respectively. They can be recovered from  the anode mud that is left behind after the electrolytic refining of copper metal. The coins made of metal X (gold) look new even after several years of use because gold is the least reactive metal and it does not react with air, water, acids, alkalis etc. On the other hand, coins made of metal Y (silver) lose their shine gradually and get blackened soon because silver reacts with hydrogen sulphide gas in the air and forms a thin black layer of silver sulphide (Ag2S) on it.
When metal X (gold) is alloyed with a small amount of metal Y (silver), it becomes hard and suitable for making ornaments. Pure gold is very soft and cannot be used for making ornaments. However, when it is mixed with silver, it becomes hard and suitable for making ornaments.
The colour of metal X (gold) is yellow.



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