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Page No 59:

Question 1:

Elements are usually divided into
(a) acids and alkalis.
(b) metals and non-metals.
(c) compounds and mixtures.
(d) alloys and ores.

Answer:

(b) metals and non-metals
There are around 118 elements known to us from the periodic table, which are usually divided into metals and non-metals.

Page No 59:

Question 2:

Which one of the following is a non-metallic element?
(a) mercury
(b) sulphur
(c) water
(d) carbon dioxide

Answer:

(b) Sulphur
Sulphur is a non-metal; mercury is metal; water and carbon dioxide are compounds.

Page No 59:

Question 3:

Which of the following compounds can be produced by the combination of a metal with an acid?
(a) iron chloride
(b) magnesium oxide
(c) copper sulphide
(d) carbon dioxide

Answer:

(a) iron chloride

Generally, metals react with acids to form the salt of the acid and hydrogen gas. Iron reacts with hydrochloric acid to form iron chloride (salt) and hydrogen gas.

Page No 59:

Question 4:

The formation of calcium oxide from its elements is a reaction between
(a) two metals.
(b) two non-metals.
(c) a metal and a non-metal.
(d) an alkali and an acid.

Answer:

(c) a metal and a non-metal

Calcium, which is a metal, reacts with oxygen, which is a non-metal, to form calcium oxide.



Page No 60:

Question 5:

Radhika carried out tests on four elements W, X, Y and Z. The results are shown in the table below.

Element Solid at room temperature Conducts electricity Can be hammered into sheets
W No No No
X Yes No No
Y Yes Yes No
Z Yes Yes Yes

From these results, which of the four elements could she say is a metal?
(a) W
(b) X
(c) Y
(d) Z

Answer:

(d) Z

Most of the metals are solid at room temperature, conduct electricity and are malleable in nature, i.e. they can be hammered into sheets. Element 'Z' satisfies all these properties.

Page No 60:

Question 6:

Which sequence indicates correctly the increasing reactivity of metals?
(a) iron, zinc, magnesium, sodium, potassium
(b) sodium, potassium, magnesium, zinc, iron
(c) magnesium, potassium, sodium, iron, zinc
(d) iron, zinc, sodium, magnesium, potassium

Answer:

(a) iron, zinc, magnesium, sodium, potassium

The reactivity of the metals decreases down the reactivity series. In the reactivity series, the given metals are arranged as: (more reactive) potassium, sodium, magnesium, zinc, iron (less reactive). Thus, option (a) correctly indicates the increasing reactivity of the metals.

Page No 60:

Question 7:

Which metal from this list will displace all the other from solutions of their salts?
(a) copper
(b) iron
(c) magnesium
(d) zinc

Answer:

(d) magnesium

Magnesium is more reactive than zinc, iron and copper. It is placed above these metals in the reactivity series and can displace them from the solutions of their salts.

Page No 60:

Question 8:

Dr Kumar is trying to identify an unknown metal Z. When he places it in copper sulphate solution, there is a reaction and red brown pieces of copper fall to the bottom the test tube. When he placed it in magnesium sulphate solution, nothing happens. Which one of the elements given below do you think it is?
(a) sodium
(b) iron
(c) calcium
(d) barium

Answer:

(b) iron

The unknown metal Z should be iron. Among the given choices, only iron is less reactive than magnesium. Thus, iron, (placed above copper in the reactivity series) being more reactive than copper, displaces copper from the copper sulphate solution. But, iron, (placed below magnesium in the reactivity series) being less reactive than magnesium, cannot displace magnesium from its magnesium sulphate solution.

Page No 60:

Question 9:

Which of the following reactions will take place?
A. Mg + ZnSO4 → MgSO4 + Zn
B. Zn + CuCl2 → ZnCl2 + Cu
C. Cu + FeSO4 → CuSO4 + Fe
D. Fe + MgCl2 → FeCl2 + Mg

(a) C and D
(b) A and B
(c) A, B and C
(d) all the above

Answer:

(b) A and B

Magnesium, which is more reactive than zinc, displaces zinc from its solution (reaction A). Similarly, zinc, which is more reactive than copper, displaces copper from its copper chloride solution (reaction B). But, reactions C and D cannot occur. Copper, which is less reactive than iron, cannot displace iron from its salt solution. Again, iron, which is less reactive than magnesium, cannot displace magnesium from its salt solution.

Page No 60:

Question 10:

Which of the following will be a displacement reaction?
(a) zinc + hydrochloric acid
(b) iron + copper sulphate
(c) magnesium + sulphuric acid
(d) all the above

Answer:

(d) all the above

Magnesium and zinc (placed above hydrogen in the reactivity series) are more reactive than hydrogen. Thus, they displace hydrogen from dilute acids. Iron (placed above copper in the reactivity series) is more reactive than copper. Hence, it displaces copper from its salt solution. Thus, all the given reactions are examples of displacement reaction.

Page No 60:

Question 11:

Define the following terms:
electroplating, rusting, galvanisation

Answer:

Electroplating is the process of covering a thin layer of one metal over another with the help of electricity. Electroplating is one of the methods used to protect a metal from corrosion. It gives the electroplated metal a shine and a nice look. ​For example, iron can be covered with a thin layer of chromium or tin with the help of electroplating.

Corrosion of iron is called rusting. Rusting is a process in which iron metal reacts with moist air (water + oxygen) present in the atmosphere to form iron oxide, Fe2O3, which is reddish brown in colour. Rusting of iron leads to very heavy damage to the substance. Thus, it should be prevented. Applying oil, grease or paint to iron objects can prevent rusting to some extent. Electroplating or galvanizing iron objects prevents them from rusting.

Galvanisation is the process of coating a thin layer of zinc on iron to prevent it from rusting. Objects made of iron are dipped in molten zinc to cover them with a thin layer of zinc. Iron sheets, dustbins, etc. are galvanised to prevent corrosion.

Page No 60:

Question 12:

Name the following:
(a) a non-metal present in steel
(b) a metal which reacts vigorously with cold water
(c) a non-metal used to vulcanise rubber
(d) a metal stored in oil
(e) a metal that becomes black in the presence of hydrogen sulphide
(f) the gas released when metals react with water
(g) a metal that replaces silver from silver nitrate
(h) a metal used for galvanising iron
(i) a metal that will displace copper from copper sulphate solution
(j) a noble metal

Answer:

(a) The non-metal present in steel is carbon. 
(b) A metal which reacts vigorously with cold water is sodium. 
(c) A non-metal used to vulcanise rubber is sulphur. 
(d) A metal stored in oil is sodium.
(e) A metal that becomes black in the presence of hydrogen sulphide is silver.
(f) Hydrogen gas is released when metals react with water.
(g) A metal that replaces silver from silver nitrate is copper.
(h) A metal used for galvanising iron is zinc.
(i) A metal that will displace copper from copper sulphate solution is iron.
(j) Gold is a noble metal.



Page No 61:

Question 13:

An archaeologist found a gold coin, a silver bracelet and an iron sword while digging at a site. On washing them carefully, he found that while the gold coin shone brightly, the iron sword had a reddish brown colour and was chipped from the sides and the silver bracelet had turned black. Explain these observations.

Answer:

The observations made by the archaeologist can be explained as follows:
Gold, being a noble metal, is unreactive towards air, water, acids and other gases in the atmosphere. It does not corrode and retains its shine for a very long period of time. Thus, the gold coin had a bright appearance even though it was buried underground. Although silver is less reactive and does not react with the oxygen present in the air, it reacts with the hydrogen sulphide present in the air forming a thin layer of silver sulphide on its surface. Thus, the silver bracelet had turned black. Iron, being highly reactive, reacts with moist air (water + oxygen) to form iron oxide, a reddish brown powdered layer called rust, on its surface. The iron sword had reacted with moist air and had undergone corrosion.

Page No 61:

Question 14:

Give two uses each of the following metals:
iron, copper, aluminium

Answer:

Uses of iron: 
1. Iron is specifically used to prepare magnets.
2. Alloy of iron, stainless steel does not rust; therefore, it is widely used for making utensils for cooking.

Uses of copper:
1. Copper is used for making electrical wires.
2. Alloy of copper, brass is resistant to corrosion and has a golden colour; therefore, it is used for making decorative items, statues, etc.

Uses of aluminium:
1. Aluminium is widely used for making tins, cans, foils, utensils, etc.
2. Alloy of aluminium, duralumin is used to make bodies of aircrafts because of its light weight, strength and resistance to corrosion.

Page No 61:

Question 15:

Alloys have some advantages over a pure metal. Explain this statement with two examples.

Answer:

We know that an alloy is a uniform mixture of two or more metals or a metal with a non-metal. An alloy usually has different properties from that of the pure metal with which it is made. Alloys have some advantages over pure metals. Let us explain this with two examples.
1. Stainless steel, an alloy of iron with nickel and chromium, is much stronger than iron and does not corrode at all. Thus, stainless steel has replaced iron in the manufacture of utensils, nettings, etc.
2. Brass (copper + zinc) and bronze (copper + tin) are alloys of copper. They are harder, stronger and have higher resistance to corrosion as compared to copper. Thus, these alloys are used for making statues, medals, decorative articles, etc.

Page No 61:

Question 16:

Give the differences between the properties of metals and non-metals with suitable examples.

Answer:

The differences between the properties of metals and non-metals are explained below with suitable examples.
 

Metal Non-metal
1. Metals are generally solid at room temperature. Mercury is an exception, which is liquid at room temperature. Non-metals are solid, liquid and gaseous at room temperature. For example, carbon and sulphur are solids; bromine is a liquid; oxygen, chlorine and nitrogen are gases.
2. Metals are usually hard and strong. They can resist high weight without breaking. Exceptions are sodium and potassium, which are soft metals and can be cut through with a knife. Generally, non-metals are not strong and they break easily. Diamond, which is an exception, is a form of carbon and is the hardest identified material.
3. Generally, metals are malleable and ductile in nature. They can be beaten into thin sheets (malleability) and can be drawn into very thin wires (ductility). For example, gold and silver are highly malleable metals. And, copper, aluminium, gold and silver are highly ductile metals. Generally, non-metals are not malleable and ductile in nature. They are highly brittle.
4. Most of the metals are lustrous. They have a shiny appearance and can be polished to bring back the shine. Gold, silver and platinum are the most lustrous metals. Non-metals are usually dull and cannot be polished. Graphite and iodine, however, are non-metals with a shiny look.
5. Most of the metals are sonorous. When these metals are hit, they make a ringing sound. For example, bells in schools and temples are made from copper alloys. Non-metals are not sonorous.
6. Generally, metals are good conductors of heat and electricity. For example, silver is the best conductor of heat and electricity. Copper and aluminium are also good conductors. Thus, metals are used for making electrical wires. Generally, non-metals are bad conductors of heat and electricity. Graphite is an exception.
7. Generally, metals have high melting and boiling points.  Generally, non-metals have low melting and boiling points. 

Page No 61:

Question 17:

Give reasons for the following:
(a) Steel is used to make cooking vessels.
(b) Gold is found in nature as an element in the free state.
(c) Titanium is used to make the body of a spacecraft.
(d) An alloy of gold is used in dentistry.
(e) Silver is used for making mirrors.
(f) Iron sheets are galvanised before use.
(g) Aluminium does not corrode easily.

Answer:

(a) Steel is an alloy of iron, but it has resistance to corrosion. Thus, it is used to make cooking vessels.

(b) Gold is a non-reactive metal and does not react with atmospheric gases and water. Therefore, gold is found in nature as an element in the free state.

(c) Titanium is a light metal with high strength. It can sustain very high temperatures. Hence, titanium is used to make the body of a spacecraft.

(d) Gold is a non-poisonous, safe metal. It does not react with acids. It is highly malleable and ductile in nature. Thus, it is suitable for use in dentistry.

(e) Among the known metals, silver has the best reflecting capability. Therefore, it is used for making mirrors.

(f) Iron is a highly reactive metal. It reacts with moist air (water + oxygen) and forms iron oxide, a reddish-brown powdered layer called rust, on its surface. Corrosion of iron is called rusting. To avoid this, iron sheets are galvanised (iron objects are coated with zinc layer) before use.

(g) Aluminium is a highly reactive metal. It reacts easily with oxygen present in the air, forming a layer of aluminium oxide on its surface. This layer of aluminium oxide sticks to the metal and protects it. Thus, aluminium articles become unreactive towards air and water, and do not corrode easily due to the presence of this strong aluminium oxide layer over them. 

Page No 61:

Question 18:

Arrange the following in the order of decreasing reactivity of metals:
magnesium, potassium, iron, gold

Answer:

Potassium
Magnesium
Iron
Gold

Page No 61:

Question 19:

Give three methods by which rusting can be prevented.

Answer:

Corrosion of iron is called rusting. Three methods by which rusting can be prevented are mentioned below:

1. Applying a layer of paint, oil or grease prevents the contact of air and moisture with the metal surface, thereby preventing rusting of iron objects. Therefore, iron objects like grills, railings, pipes, bridges, railway coaches, buses, cars, trucks, etc. are usually painted.

2.  Electroplating is another method used to prevent iron from rusting. In electroplating, a thin layer of one metal is coated over another metal with the help of electricity. Using this method, iron can be covered with a thin layer of chromium or tin. Iron coated with tin (non-reactive metal) is used to make food storage cans. Iron coated with chromium is used to make taps, kettles, car bumpers, etc.

3. Galvanisation is the process of coating a thin layer of zinc over iron to prevent it from rusting. The iron objects are dipped in molten zinc to get them covered with a thin layer of zinc. Iron sheets, dustbins etc., are galvanised to prevent corrosion of iron.

Page No 61:

Question 20:

The cans used for soft drinks are made of aluminium.
(a) Give two reasons why aluminium is a good material for this purpose.
(b) Explain why it is important to recycle these cans.

Answer:

(a) The cans used for soft drinks are made of aluminium because of the following reasons:
     (i) Aluminium is light weight, strong and highly corrosion resistant.
     (ii) Aluminium cans are recyclable.

(b) It is very important to recycle these aluminium cans as they cause waste disposal problems. Also, aluminium is non-biodegradable. As the strength or quality of the metal is not affected by recycling, we can recycle it as many times as we want. This saves our limited and precious natural resources.

Page No 61:

Question 21:

Explain what difference you would see when zinc is put into hydrochloric acid and when sulphur is put into hydrochloric acid.

Answer:

Generally, metals react with acids to form salts of the acids along with the liberation of hydrogen gas. But, non-metals are non-reactive towards dilute acids. Thus, when zinc is put into hydrochloric acid, it reacts with the acid producing zinc chloride salt along with the evolution of hydrogen gas.

      Zn     +         2HCl                     ZnCl2    +    H2    Zinc          Hydrochloric acid        Zinc Chloride     Hydrogen gas

When sulphur is put into hydrochloric acid, no reaction takes place.

       S     +           HCl            No Reaction    Sulphur    Hydrochloric acid

Page No 61:

Question 22:

Shyama performed some displacement reactions between metals and some metallic salts. Copy and complete the table to show her findings. Put a cross wherever you think a reaction is not possible. In case it is possible, write the name of the salt formed.

Metal Solution to Which Metal is Added
Magnesium
Chloride
Iron
Sulphate
Lead
Nitrate
Copper
Sulphate
Silver
Nitrate
magnesium          
iron          
lead          
copper          
silver          

Answer:

Metal Solution to Which Metal is Added  
Magnesium
Chloride
Iron
Sulphate
Lead
Nitrate
Copper
Sulphate
Silver
Nitrate
Magnesium X Magnesium Sulphate Magnesium Nitrate Magnesium Sulphate Magnesium Nitrate
Iron X X Iron Nitrate Iron Sulphate Iron Nitrate
Lead X X X Lead Sulphate Lead Nitrate
Copper X X X X Copper Nitrate
Silver X X X X X



Page No 62:

Question 23:

Look at the diagram of an experiment shown here and answer the questions.
Figure

(a) What are the reactants?
(b) Name the gas that is evolved in this reaction.
(c) Write a balanced chemical equation for this reaction.

Answer:

(a) The reactants are calcium and dilute hydrochloric acid.
(b) Hydrogen is the gas evolved in this reaction.

(c) The balanced chemical equation is:

    2Ca   +       dil. 2HCl                 CaCl2        +      H2Calcium     Hydrochloric acid       Calcium chloride   Hydrogen gas

Page No 62:

Question 24:

Compare the following pairs of reactions, giving equations in each case.
(a) the reaction of sodium and calcium with water
(b) the reaction of aluminum and silver with atmospheric oxygen
(c) the reaction of zinc and copper with dilute sulphuric acid

Answer:

(a) Sodium, being highly reactive, reacts vigorously with water forming sodium hydroxide and hydrogen gas with excess of heat generation.

   2Na  +  2H2O            2NaOH        +      H2 (Excess heat liberated)Sodium     Water            Sodium hydroxide      Hydrogen gas

The reaction of calcium with water is not as violent as the reaction of water with sodium. Calcium reacts with water forming calcium hydroxide and hydrogen gas with very less heat generation.

     Ca   +   2H2O            Ca(OH)2       +     H2 (less heat liberated)Calcium     Water           Calcium hydroxide   Hydrogen gas

(b) Aluminium is a reactive metal and reacts easily with the atmospheric oxygen forming a thin layer of aluminium oxide on its surface. 

      4Al       +  3O2         2Al2O3   Aluminium    Oxygen       Aluminium oxide

Silver, being less reactive, does not react with oxygen in present in the atmosphere, but it reacts with hydrogen sulphide present in the air forming a thin layer of black silver sulphide on its surface.


(c) Zinc, being highly reactive, reacts with dilute sulphuric acid and displaces hydrogen from the acid forming zinc sulphate salt and hydrogen gas.

    Zn   +    dil. H2SO4                ZnSO4       +     H2  Zinc      dil. Sulphuric acid        Zinc sulphate    Hydrogen gas

Copper, being a less reactive metal, does react with dilute sulphuric acid.

     Cu    +     dil. H2SO4         No Reaction  Copper       dil. Sulphuric acid

Page No 62:

Question 25:

Write balanced chemical equations for the following reactions.
(a) the action of water on calcium oxide
(b) the action of dilute nitric acid on zinc
(c) the reaction between sulphur dioxide and water
(d) the reaction between sodium and water
(e) the reaction between calcium and sulphuric acid
(f) the reaction between iron and copper sulphate solution
(g) the reaction between potassium and water

Answer:

(a) Calcium oxide reacts with water to form calcium hydroxide along with liberation of excess heat. The balanced chemical equation is:

         CaO       +    H2O           CaOH2    +   Excess heatCalcium Oxide     Water             Calcium hydroxide

(b) Zinc reacts with dil. nitric acid to form zinc nitrate along with the evolution of nitrous oxide. The balanced chemical equation is:​

    4Zn + dil. 10HNO3        4ZnNO32   +     N2O     +  5H2O   Zinc    dil. Nitric acid            Zinc nitrate        Nitrous oxide   Water

(c) Sulphur dioxide reacts with water forming sulphurous acid. The balanced chemical equation is:​

           SO2        +  H2O             H2SO3Sulphur dioxide     Water         Sulphorous acid

(d) Sodium reacts with water violently forming sodium hydroxide and hydrogen gas with an excess of heat generation. The balanced chemical equation is:​

  2Na  +  2H2O            2NaOH       +     H2    (Excess of heat)Sodium      Water          Sodium hydroxide    Hydrogen gas

(e) Calcium reacts with sulphuric acid forming calcium sulphate and hydrogen gas. The balanced chemical equation is:​​

     Ca   +     H2SO4            CaSO4      +     H2  Calcium   Sulphuric acid      Calcium sulphate   Hydrogen gas

(f) Iron being more reactive than copper displaces copper from copper sulphate solution forming iron sulphate. The balanced chemical equation is:​​

     Fe   +     CuSO4             FeSO4      +     Cu   Iron       Copper sulphate      Iron sulphate     Copper

(g) Potassium reacts with water violently forming potassium hydroxide and hydrogen gas with excess of heat generation. The balanced chemical equation is:​​

      2K  +  2H2O            2KOH       +           H2    (Excess of heat)Potassium     Water        Potassium hydroxide    Hydrogen gas

Page No 62:

Question 26:

Study the table below and answer the questions that follow.

Metal Reactivity with
Water Air Dilute Acids
A does not react does not react dilute acids
B mild reaction with
steam
develops coating of
oxide if exposed to air
will react to form salt of
acid and hydrogen
C vigorous vigorous vigorous
D can react can react will react to form salt of
acid and hydrogen

(a) Which of these elements will react with cold water to form hydrogen gas?
(b) Which of these will rust?
(c) Which is an unreactive metal?
(d) Arrange the metals shown above in the order of decreasing reactivity.

Answer:

(a) Metals C and D will react with cold water to form hydrogen gas.
(b) Metal B will rust.
(c) Metal A is an unreactive metal.
(d) The given metals can be arranged in the order of decreasing reactivity as shown below:
Metal C (highly reactive)
​Metal D (reactive)
Metal B (moderately reactive)
Metal A (least reactive)
The reactivity of metals decreases from C to A.



Page No 63:

Question 27:

Copy the chart shown here in your notebook and fill in at least two examples in the blank spaces provided at each step of the chart.
Figure

Answer:

The completed chart, with examples, is given below:



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