Living Science 2019 Solutions for Class 8 Science Chapter 4 Metals And Non Metals are provided here with simple step-by-step explanations. These solutions for Metals And Non Metals are extremely popular among Class 8 students for Science Metals And Non Metals Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Living Science 2019 Book of Class 8 Science Chapter 4 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Living Science 2019 Solutions. All Living Science 2019 Solutions for class Class 8 Science are prepared by experts and are 100% accurate.

Page No 56:

Question 1:

A rock is found to have a very small amount of iron. Geologists thought it would be too expensive to extract iron from it. would you call the rock an ore of iron?

Answer:

An ore is defined as a mineral from which metal can be extracted economically and profitably. So, the rock obtained by geologists cannot be considered as an ore because it's too expensive to extract iron from it.

Page No 56:

Question 2:

An element is a metal or not is finally determined by whether it is hard and shiny or not. Do you agree? Give reasons.

Answer:

Yes, I agree. Generally, all metals are hard and shining, except few exceptions like sodium which can be cut through a knife and zinc which can break even with a little pressure. On the other hand, non-metals don't shine and are not much hard. So, we can know if a substance is a metal or not by determining if it is hard and shining or not.
 

Page No 56:

Question 3:

Which property must a substance possess for it to be possible to make thin wires of the material?            

Answer:

A substance(normally metals) should be ductile in nature, for it to be possible to make thin wires of the material. Most of the metals have this property of ductility, that's why they can be stretched into wires. e.g. iron, copper, etc.

Page No 56:

Question 4:

A metal was burnt in air. The Substance formed was dissolved in water and tested with blue and red litmus paper. What result do you expect?

Answer:

M+O2MO2

When a metal is burnt in the air, it gets oxidized in the presence of oxygen and forms metal oxide. The metal oxide is basic in nature, so it produced a basic solution when dissolved in water. Hence, in the basic solution, blue litmus paper will not change its colour whereas the red litmus will turn blue. 

Page No 56:

Question 5:

Metal M1 is more reactive than metal M2. Which metal will be higher up in the reactivity series?

Answer:

The reactivity series is a series of metals, which follows the order of reactivity from highest to lowest from the top to the bottom of the series. So, metals with higher reactivity are found at the top of this series. As metals M1 is more reactive than metal M2, so it would be found higher up in the reactivity series. 

Page No 56:

Question 6:

Metal M3 reacted with a salt of metal M4. The products were M4 and salt of M3. However, metal M4 had no reaction with a salt of M3. What can you say about the reactivities of M3 and M4?

Answer:

When metal M3 is reacted with a salt of metal M4 then it displaces metal M4 from its salt solution and salt of metal M3 is formed. But when metal M4 is reacted with a salt of metal M3, no reaction takes place. Hence, metal M3 is more reactive than metal M4, because it displaced metal M4 from its salt solution but metal M4 could not displace metal M3 from its salt solution. 



Page No 60:

Question 1:

Which metalloid can be said to be the backbone of the electronics industry?

Answer:

Silicon is the metalloid which is said to be the backbone of the electronics industry. Because, it is a semiconductor, which makes it most suitable for the making of various electronic devices and also it is the second most abundant element in the earth's crust, so it is easily available and affordable.

Page No 60:

Question 2:

Steel is not a pure form of iron. What does it consist of? What is the advantage of adding another substance to iron?

Answer:

Steel is an alloy of iron and carbon. Carbon is added to the iron because it provides strength to the iron and protects it from rusting.

Page No 60:

Question 3:

Noble Metals are used to make jewellery because they are very reactive. Do you agree? Give reasons.

Answer:

No, I do not agree. Noble metals like gold, platinum, and silver are used to make jewelry, not because they are very reactive but because they are inert(Less reactive) in nature and highly resistant to corrosion.

Page No 60:

Question 4:

Painting a metal prevents it from corroding. Why?

 

Answer:

When a metal reacts with the oxygen and water(moisture) present in the atmosphere, then a coating of metal oxide is formed on the surface of the metal, which corrodes the metal. Painting of metal cuts of the moisture and oxygen from the metal surface and stops the reaction from happening, therefore, painting a metal prevents it from corroding.



Page No 62:

Question 1:

Which of the following is true for all metals?
(a) They are hard solids.
(b) They have high melting and boiling points.
(c) They have 1−3 valence electrons.
(d) They are malleable and ductile.

Answer:

(c) They have 1-3 valence electrons.
All the metals have 1-3 electrons in their valence shell and they lose their valence electrons to form positive ions. However, other properties like hardness, high melting and boiling points, malleability and ductility are not shown by all metals. Even though sodium and potassium are solids, they are soft, non-malleable and non-ductile metals. Further, they have low melting and boiling points. Similarly, mercury is found in liquid state at room temperature.

Page No 62:

Question 2:

Gold normally occurs in nature
(a) in free state.
(b) as carbonate ore.
(c) as sulphate ore.
(d) as sulphide ore.

Answer:

(a) in free state
Gold is an inert metal and doesn't react easily, therefore, it occurs in free state in nature.

Page No 62:

Question 3:

Which of these is the most reactive metal?
(a) magnesium
(b) gold
(c) platinum
(d) sodium

Answer:

(d) Sodium
Sodium reacts vigorously with oxygen and water at room temperature by catching fire. It is considered highly reactive and placed at the top in the reactivity series. It is stored under kerosene to prevent reaction.

Page No 62:

Question 4:

Which of these metals cannot displace hydrogen from a dilute acid?
(a) iron
(b) zinc
(c) silver
(d) calcium

Answer:

(c) silver

Silver cannot displace hydrogen from a dilute acid because it is a less reactive noble metal and does not react with acids.

Page No 62:

Question 5:

Which one of the following alloys is light and strong?
(a) brass
(b) stainless steel
(c) duralium
(d) bronze

Answer:

(c) duralumin

Duralumin contains aluminium, copper, magnesium and manganese. These constituents make this alloy light and strong. Therefore, it is used in the construction of airplane and vehicle bodies. 

Page No 62:

Question 6:

Which of these metals can displace all the other three metals from their salt solutions?
(a) copper
(b) iron
(c) silver
(d) zinc

Answer:

(d) zinc
Zinc is more reactive than the other three metals; therefore, it can replace these metals from their salt solutions. In the reactivity series, iron, copper and silver are placed below zinc. Thus, they are less reactive than zinc and are displaced by it from their salt solutions.

Page No 62:

Question 7:

Metal A dipped in a salt of metal B displaces B from the solution. This shows that
(a) A is more reactive than B.
(b) B is more reactive than A.
(c) both A and B are equally reactive.
(d) no such conclusion can be drawn.

Answer:

(a) A is more reactive than B
More reactive metal (A) can displace a less reactive metal (B) from its salt solutions. More reactive metals are placed above comparatively less reactive metals in the reactivity series and can displace metals below them in the reactivity series.

Page No 62:

Question 8:

Metals react with oxygen to form
(a) acidic oxides.
(b) basic oxides.
(c) either acidic or basic oxide depending on the metal.
(d) there is no reaction

Answer:

(b) basic oxides
Usually, most of the metals react with oxygen to form basic oxides. Like bases, these metal oxides also react with acids in the same way and produce salt and water.

Page No 62:

Question 9:

The metal most used in the construction industry is
(a) iron.
(b) copper.
(c) aluminium.
(d) tin.

Answer:

(a) iron
Steel, an alloy of iron and carbon (less than 1%), is very strong and widely used in the construction industry.



Page No 63:

Question 10:

Pieces of copper, silver and gold are dropped into a solution of iron sulphate. The piece that will get a coating of copper is
(a) iron.
(b) silver.
(c) gold.
(d) none of them

Answer:

(d) none of them
All the three metals - copper, silver and gold - are less reactive than iron and are placed below iron in the reactivity series. Therefore, none of the three metals can displace iron from its salt solution. Hence, we observe that no reaction takes place and none of the pieces get coated.

Page No 63:

Question 1:

Name a metal which is liquid at room temperature.

Answer:

Mercury (Hg) is liquid at room temperature.

Page No 63:

Question 2:

Metals generally have a low specific gravity. True or false?

Answer:

False.
Metals generally have a very high specific gravity. Metals like iron, lead, mercury, gold etc. have specific gravity values greater than 7.5. However, sodium and potassium are the only metals with a specific gravity value below 1. 

Page No 63:

Question 3:

If a material can be drawn into thin wires, it is said to be __________

Answer:


If a material can be drawn into thin wires, it is said to be ductile.

Page No 63:

Question 4:

Aluminium does not react with water. True or false?

Answer:

False.
Aluminium reacts with water to form a thin layer of aluminium oxide on it. This thin coating of aluminium oxide on aluminium acts as a barrier and stops further reactions.

Page No 63:

Question 5:

Zn + CuSO4 → ZnSO4 + Cu is an example of a __________ reaction.

Answer:

Zn + CuSO4 → ZnSO4 + Cu is an example of a displacement reaction.

Page No 63:

Question 6:

Which metal is higher up in the reactivity series − sodium or silver?

Answer:

Sodium, being highly reactive metal, is placed high in the reactivity series, whereas silver, being very less reactive metal, is placed low in the reactivity series.

Page No 63:

Question 7:

Any rock containing metallic mineral is called an ore. True or false?

Answer:

False. Any rock containing metallic mineral is not an ore as it depends on the quantity of metal in the rock. If the quantity of metal in the rock is less, it cannot be extracted profitably. Thus, rocks containing metallic minerals in adequate quantity, from which metals can be extracted profitably, are called ores.

Page No 63:

Question 8:

__________ is the most abundant element in the universe, whereas __________ is the most abundant element in the earth's crust.

Answer:


Hydrogen is the most abundant element in the universe, whereas oxygen is the most abundant element in the earth's crust.

Page No 63:

Question 9:

Sand contains the non-metals __________ and __________

Answer:

Sand contains the non-metals silicon and oxygen.

Page No 63:

Question 10:

Which non-metal has very high melting and boiling points?

Answer:

Graphite is a non-metal, which has very high melting and boiling points.

Page No 63:

Question 11:

Metal food containers have a coating of __________

Answer:

Metal food containers have a coating of tin (Sn).

Page No 63:

Question 12:

Name a soft metal.

Answer:

Sodium is a soft metal, which can be cut easily with the help of a knife.

Page No 63:

Question 13:

What is the black material inside a pencil?

Answer:

The black material inside a pencil is graphite, an allotrope of carbon (non-metal).

Page No 63:

Question 14:

Metals react with oxygen to produce __________ oxides, and non-metals react with oxygen to produce __________ oxides.

Answer:

Most metals react with oxygen to produce basic oxides and non-metals react with oxygen to produce acidic oxides.

Page No 63:

Question 15:

Do non-metals react with water?

Answer:

No, non-metals do not react with water. Hence, highly reactive non-metals such as phosphorous, which catch fire when exposed to air, are stored under water.

Page No 63:

Question 16:

A metal X can replace another metal Y from its metal salt. Is X above or below Y in the reactivity series?

Answer:

X is above Y in the reactivity series. A more reactive metal can displace a less reactive metal from its salt solution. More reactive metals are placed at the top of the reactivity series and can displace other metals below them in the reactivity series.

Page No 63:

Question 17:

The presence of air and __________ are necessary for iron to rust.

Answer:

The presence of air and moisture are necessary for iron to rust.

Page No 63:

Question 18:

Galvanized iron starts rusting if there is a scratch on the zinc layer. True or false?

Answer:

False.
Galvanised iron does not rust even if there is a scratch on the zinc layer. Even if there is a scratch, zinc reacts with atmosphere and forms sacrificial layer of zinc carbonate, which provides protection against corrosion.

Page No 63:

Question 19:

Copper corrodes in the presence of __________ and water.

Answer:

Copper corrodes in the presence of carbon dioxide and water.

Page No 63:

Question 20:

Name a metal that does not corrode in air.

Answer:

Gold does not corrode in air. It is the least reactive metal. Hence, gold jewellery glitters even after many years.

Page No 63:

Question 1:

What is the difference between the physical state of metals and non-metals at room temperature?

Answer:

Physical state of metals at room temperature Physical state of non-metals at room temperature
Metals are usually in solid state at room temperature except mercury and gallium, which are in liquid state at room temperature. Non-metals occur in all the three states, i.e., solid (sulphur, phosphorous), liquid (bromine) and gas (oxygen, nitrogen, chlorine) at room temperature.

Page No 63:

Question 2:

What do you mean by reactivity series of metals?

Answer:

Metals' reactions with oxygen, water and acids prove that they differ in reactivity. Thus, they can be arranged in an order of their reactivity. The series of arrangement of metals in an order of decreasing reactivity is called reactivity series. Highly reactive metals (potassium, sodium) are placed at the top of the reactivity series, whereas least reactive metals (silver, gold) are placed at the bottom of the reactivity series.

Page No 63:

Question 3:

What is the advantage of galvanized iron over tin-plated iron?

Answer:

Advantage of galvanised iron over tin-plated iron is that galvanised iron does not rust even if there is scratch on the coating of zinc layer on iron surface. This is because zinc is more reactive than iron and reacts with air to form a sacrificial layer of zinc carbonate, which provides protection against corrosion of iron. In case of tin-plated iron, it easily begins to rust at the place where there is scratch on the tin-plated layer. This is because iron is more reactive than tin and it undergoes oxidation in preference to tin. 

Page No 63:

Question 4:

What is an alloy? How are alloys generally made?

Answer:

Alloy is a mixture (or a solid solution) of two or more metals, or one or more metals along with a non-metal.
Alloys are generally made by combining two metals in their molten state. Sometimes, they are also made by combining the powders of two metals.

Page No 63:

Question 5:

What are noble metals? Why are they used to make ornaments?

Answer:

Metals, which are unreactive and present in free state in nature, are called noble metals. These metals are placed at the bottom of the reactivity series. Silver, gold and platinum are called noble metals.
Noble metals do not react with water, acids and alkalis and are unreactive in nature. Therefore, noble metals are mainly used for making ornaments as these ornaments retain their shine for a long time.

Page No 63:

Question 6:

Can a wire be drawn out of wood. Why not?

Answer:

No, a wire cannot be drawn out of wood. Wires can be drawn out from ductile substances like metals. As wood is highly brittle in nature and a non-ductile substance, it cannot be drawn into thin wires.

Page No 63:

Question 7:

Can alloying be used to lower the melting point? Give an example.

Answer:

Yes, alloying can be used to lower the melting point. Solder, an alloy of tin and lead, has a low melting point than that of tin and lead. Thus, this property of solder is helpful for copper wires to be used as a fuse in an electrical circuit. Hence, alloying helps to change the properties of a metal.

Page No 63:

Question 8:

How is anodizing of aluminium carried out?

Answer:

Aluminium reacts with air and forms a thin layer of aluminium oxide on it. This layer of oxide prevents further reactions and protects the metal from corrosion. If this oxide layer formation is more uniform, the metal below the layer would be well-protected. This can be formed with the help of a process called anodising. In this process, electric current is passed through sulphuric acid, utilising aluminium as anode. Oxygen is liberated at anode and it reacts with aluminium and forms a uniform oxide layer of aluminium, which protects the metal from corrosion. This is how anodising of aluminium is done.



Page No 64:

Question 1:

What do you mean by malleability and ductility? How are these properties different in metals and non-metals?

Answer:

The property of a substance to be hammered into thin sheets is called malleability. The property of a substance to be stretched into thin wires is called ductility. The property of malleability and ductility differs in metals and non-metals. Most of the metals have high degree of malleability and ductility; for example, aluminium, copper, gold, etc. However, non-metals are highly brittle and do not exhibit these properties; for example, coal and graphite (both contain carbon).

Page No 64:

Question 2:

Give three examples of reactions between metals and water, to show that different metals have different reactivities.

Answer:

Metals react with water to form oxides or hydroxides with release of hydrogen gas.

1. Sodium and potassium react vigorously even with cold water producing hydroxides with release of enormous heat.
 
   2 NaSodium + 2 H2OWater  2 NaOHSodium hydroxide + H2Hydrogen2 KPotassium + 2 H2OWater  2 KOHPotassium hydroxide + H2Hydrogen      
                       
2. Magnesium does not react with cold water, but reacts with hot water or steam to form magnesium oxide.
     
 
2 MgMagnesium + 2 H2OWater  2 MgOMagnesium oxide + 2 H2Hydrogen
                                       
3.
Aluminium reacts with water to form a thin layer of aluminium oxide on it. This thin coating of aluminium oxide on aluminium acts as a barrier and stops further reactions.

2 AlAluminium + 3 H2OWater  Al2O3Aluminium oxide + 3H2Hydrogen

From the above examples, we can say that sodium and potassium are highly reactive, magnesium is less reactive and aluminium is least reactive. This shows that different metals have different reactivities with water.

Page No 64:

Question 3:

Give the important uses of copper.

Answer:

Important uses of copper are:
i. Copper is commonly used in electric cables and wires. It is also used as coil in many electrical appliances.
ii. It is used to prepare utensils, calorimeters and car radiators.
iii. It is also used to form alloys such as brass, bronze, which are hard and corrosion resistant with many uses.

Page No 64:

Question 4:

Aluminium protects itself against corrosion. Discuss why it is necessary to anodize aluminium to make it corrosion resistant.

Answer:

Aluminium reacts with air and forms a thin layer of aluminium oxide on it. This layer of oxide prevents further reactions and protects the metal from corrosion. This is how aluminium protects itself against corrosion. 
4 Al + 3 O2  2 Al2O3
But this oxide layer is not uniform and may lead to corrosion over a period of time. If it is more uniform, the metal below the layer would be well-protected. This can be made with the help of a process called anodising. In this process, electric current is passed through sulphuric acid, utilising aluminium as anode. Oxygen, which is liberated at anode, reacts with aluminium and forms a uniform oxide layer of aluminium, which protects the metal from corrosion. This is why anodising of aluminium is essential.

Page No 64:

Question 5:

List three important uses of hydrogen.

Answer:

Three important uses of hydrogen are:
i. Hydrogen is mainly used in the manufacture of ammonia, which is further used to prepare fertilisers like urea, ammonium sulphate etc.
ii. It is used in the extraction of metals such as copper, lead, tungsten, tin; for example, hydrogen is used as a reducing agent in the reduction of tungsten oxide to purify tungsten during extraction.
iii. It is used in cutting and welding of metals in industries. When hydrogen is burnt in oxygen, an oxy-hydrogen flame is obtained by which cutting and welding of metal is performed. 

Page No 64:

Question 6:

List three uses of silicon.

Answer:

Three uses of silicon are:
i. Silicon is widely used in electronic and computer industries for the manufacturing of transistors, integrated chips and solar cells.
ii. Silicones, a compound of silicon, is used in the manufacturing of grease, polish and water-proof materials. They are also used as insulating materials in electrical equipments.
iii. Silicon carbide, a component of silicon, is used as an abrasive material.

Page No 64:

Question 1:

State whether a displacement reaction will occur if the following are mixed. Give reasons. Give the reaction.
(a) copper sulphate and zinc
(b) copper sulphate and iron
(c) zinc sulphate and copper
(d) zinc sulphate and iron
(e) iron sulphate and zinc
(f) iron sulphate and copper

Answer:

Displacement reaction is a reaction in which a one more reactive metal displaces a less reactive metal from its salt solution. They can displace other metals, which are below them in the reactivity series. Let us see if a displacement reaction takes place in the following reactions.

(a) Copper sulphate and zinc: Zinc is more reactive than copper and is placed at the top of the reactivity series above copper. Thus, a displacement reaction occurs as zinc displaces copper from copper sulphate solution.

 Zn Zinc+ CuSO4Copper sulphate  ZnSO4Zinc sulphate + CuCopper

(b) Copper sulphate and iron- Iron is more reactive than copper and is placed at the top of the reactivity series above copper. Therefore, a displacement reaction occurs as iron displaces copper from copper sulphate solution.

    FeIron + CuSO4Copper sulphate  FeSO4Ferrous sulphate + CuCopper

(c) Zinc sulphate and copper - Copper being less reactive than zinc, is placed at the bottom of the reactivity series below zinc. Therefore, a displacement reaction does not take place as copper cannot displace zinc from zinc sulphate solution

CuCopper + ZnSO4Zinc sulphate  No reaction

(d) Zinc sulphate and iron -  Iron being less reactive than zinc, is placed below zinc, in the reactivity series. Therefore, a displacement reaction does not take place as iron cannot displace zinc from zinc sulphate solution.

FeIron + ZnSO4Zinc sulphate  No reaction


(e) Iron sulphate and zinc - Zinc is more reactive than iron and is placed above iron in the reactivity series. Therefore, a displacement reaction occurs as zinc displaces iron from iron sulphate solution.

Zn Zinc+ FeSO4Ferrous sulphate  ZnSO4Zinc sulphate + FeIron

(f) Iron sulphate and copper - Copper being less reactive than iron, is placed at the bottom of the reactivity series, below iron. Therefore, a displacement reaction does not take place as copper cannot displace iron from iron sulphate solution.

 Cu Copper+ FeSO4Iron sulphate  No reaction

Page No 64:

Question 2:

Describe an experiment to show the conditions necessary for rusting of iron.

Answer:

Corrosion is the chemical destruction of metals, when they come in contact with air, water (or moisture) present in the atmosphere over a period of time. The corrosion of iron metal is called rusting. When iron undergoes oxidation by reacting with oxygen present in the air along with water (or moisture), it forms hydrated ferric (III) oxide, which is called rust.
   4Fe + 3O2 + 2xH2O  2Fe2O3.xH2O

Rusting of iron takes place in the presence of air (oxygen) along with water (or moisture).

Let us now perform an experiment which shows the necessary conditions required for rusting of iron. Take three clean test tubes and mark them as X, Y, Z. Then take three clean rust-free nails. Add a few pieces of calcium chloride in the test tube X and place some cotton over it. Now place a nail on the cotton (calcium chloride is a good absorbing agent which absorbs any water or moisture present in the test tube) and close the test tube with a cork. In the second test tube (test tube Y), add some boiled, cooled pure distilled water (boiling the distilled water for a minute removes any dissolved oxygen present in the distilled water). Place the nail in it and, to avoid contact of air, seal the surface of water by adding a layer of oil. Close the test tube with a cork. Fill half of the test tube Z with tap water and place a nail in it. Close the mouth of the test tube with a cork. Leave the three test tubes in this way for a few days and observe them. You will see that nails in test tubes X and Y are rust-free, whereas the nail in the test tube Z contains rust. This experiment shows that air (oxygen) and water (or moisture) both are required for rusting of iron.

Page No 64:

Question 3:

Discuss briefly the different methods used for preventing corrosion of metals.

Answer:

Corrosion of metals is the chemical destruction of metals, when they come in contact with air and water (or moisture) present in the atmosphere over a period of time. Examples: Rusting of iron, greenish surface of copper and black surface of silver jewels.
If corrosion is neglected, it may lead to heavy loss; therefore, corrosion should be prevented. There are different methods available to prevent corrosion. They are:
1. Applying paint, grease or oil on metal surface- Applying paint, grease or oil over the surface of the metal after thoroughly cleaning the surface is the usual way to avoid corrosion. The thick layer of paint, grease or oil prevents oxidation of metal. But once this layer is scratched or broken, the metal starts to corrode at that place.
2. Covering a metal by another metal- Covering a metal with a layer of another metal resists corrosion to a good extent. Tin, chromium, nickel and zinc are generally used to cover other metals. This can be done by following methods:
* Galvanisation: It is a process where the cleaned metal is dipped into molten zinc to get a coating of zinc layer over the metal. For example, galvanised iron pipes are widely used for water pipes. Galvanised metals do not corrode even if there is scratch on the layer of zinc. This is because zinc gives sacrificial protection to iron by reacting it with the atmosphere.
* Electroplating: It is a process by which a layer of metal is coated (using electricity) on another metal to prevent corrosion. For example, iron is electroplated by tin. It is generally used to prepare tin cans (we cannot use zinc to store food because zinc is toxic), which are used to store food materials. However, the electroplated iron corrodes if there is a scratch on the tin layer. Chrome and nickel plating are widely used for electroplating in automobile industries to prevent rust. Chrome plating not only protects the surface of metals from corrosion but also gives attractive look.
3. Anodising- This process is specially used to protect aluminium from corrosion. Aluminium reacts with air to form a thin layer of aluminium oxide on the metal This layer of oxide prevents further reactions and protects the metal from corrosion. If the formation of this oxide layer is uniform, the metal below the layer is well protected. This is done by the anodising process. In this process, electric current is passed through sulphuric acid, with aluminium as anode. Oxygen, which is liberated at anode, reacts with aluminium and forms a uniform layer of aluminium oxide, protecting the metal from corrosion.
4. Alloying- Alloy is a mixture (or a solid solution) of two or more metals or one or more metals along with a non-metal.
Alloying usually changes the properties of metals. By alloying, corrosion of metals can be prevented. For example, stainless steel (an alloy of iron, chromium and nickel) is highly corrosion resistant.

Page No 64:

Question 4:

Give the names and composition of alloys that are used for the following.
(a) making statues
(b) joining wires in electronic equipments
(c) making surgical instruments
(d) making bodies of aircrafts
(e) making accurate laboratory balances
(f) making bodies of ships

Answer:

(a) Making statues: Statues are normally made of Bronze, whose constituents are copper and tin. Because bronze is very hard and resists corrosion, it is suitable for making statues.

(b) Joining wires in electronic equipments: Solder is widely used for joining wires in electronic equipments. Solder is an alloy of tin and lead. It has low melting point which makes it suitable for fusing copper wires in an electrical circuit.

(c) Making surgical instruments: Stainless steel is widely used for making surgical instruments. Its constituents are iron, chromium and nickel. Stainless steel is very strong and resists corrosion. 

(d) Making bodies of aircrafts: Duralumin and magnalium are used for making bodies of aircraft. The constituents of duralumin are aluminium, copper, magnesium and manganese. The constituents of magnalium are aluminium and magnesium. Both duralumin and magnalium are strong as well as light. So, they are used in making bodies of aircrafts.

(e) Making accurate laboratory balances: Magnalium, an alloy of aluminium and magnesium, is widely used for making accurate laboratory balances. As it is light in weight, it is ​suitable for preparing balances.

(f) Making bodies of ships: Steel is used for making bodies of ships. The constituents of steel are iron and carbon. As steel is very strong, it is ideal for making bodies of ships.

Page No 64:

Question 1:

Silver is the best conductor of electricity. Can you say why electric wires are not made of silver?

Answer:

Silver is the best conductor of electricity, but it is highly expensive. This makes it cost-ineffective to be used in electrical wiring. Thus, electric wires are not made of silver.

Page No 64:

Question 2:

Why are bells made of metals and not from wood?

Answer:

Most of the metals are sonorous in nature; i.e., they produce ringing sound when hammered. This property of metals makes it suitable to be used in the manufacturing of bells. On the other hand, wood is not sonorous and cannot produce ringing sound when hammered. Therefore, wood it is not used in manufacturing of bells.

Page No 64:

Question 3:

Suppose copper was high up in the reactivity series. Would it still have been such a useful metal? Give reasons.

Answer:

If copper was high up in the reactivity series, it would have been a highly reactive metal. Further, it would be difficult to store it because highly reactive metals react vigorously with air, water and acids. Such a reactive metal cannot be used for preparing cooking vessels, electrical wires or all other applications, where copper is generally used.

Page No 64:

Question 4:

It has been noticed that silverware becomes black faster today than it did fifty years ago. What can be the reason for this?

Answer:

Silverwares become black due to the formation of a thin layer of silver sulphide on them by reacting with hydrogen sulphide present in the air.
2 Ag + H2S  Ag2S + H2
Fifty years ago, the presence of hydrogen sulphide in the air was very less due to pollution free environment. Thus, silver objects did not become black faster. Now a days, industrial revolution and excessive usage of automobiles have increased the percentage of sulphur in air. This has further increased the percentage of hydrogen sulphide leading to fading or blackening of silver objects faster.

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Question 5:

Aluminium is above iron in the reactivity series. However, its reactions with oxygen and water seem to indicate that it is less reactive than iron. Explain.

Answer:

Aluminium is above iron in the reactivity series. However, its reactions with oxygen and water seem to indicate that it is less reactive than iron due to the following reason:

Aluminium reacts with air and forms a thin layer of aluminium oxide on it. 
 
4 Al Aluminium+ 3 O2Oxygen  2 Al2O3Aluminium oxide2 Al Aluminium+ 3 H2OWater  Al2O3Aluminium oxide + 3 H2Hydrogen

This layer of oxide prevents further reactions, making aluminium less reactive. This layer also protects the metal from corrosion. This is how aluminium protects itself against corrosion.

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Question 6:

If you are an engineer constructing a multi-storey building. Which alloy would you be extensively using and why?

Answer:

The alloy that is used extensively during the construction of buildings is steel, which is an alloy of iron and carbon. Steel is very strong and its usage makes the building strong and sturdy.

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Question 7:

A doctor diagnosed a patient with iron deficiency, and gave him tablets containing iron. But the tablets were not hard and did not look like iron at all. What do you think the tablets contained?

Answer:

The tablets do contain iron but the iron is in the ionic form (Fe2+ as soluble ferrous ion). Such an iron in ionic form is easily absorbed by our body. It does not retain the property of hardness, which a metallic iron has. Therefore, iron tablets are not hard or look like metallic iron.



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Question 1:

Aluminium is higher than iron in the reactivity series. To see if this was true, Ayesha took similar sized pieces of aluminium and iron in test tubes. She added the same volume of dilute hydrochloric acid to each. She noticed that:
1. Initially there is more vigorous evolution of gas from the test tube having iron.
2. After a couple of minutes, aluminium starts reacting much faster than iron and evolution of gas is much more vigorous.
She was confused. Can you explain to her why this happened?

Answer:

Aluminium is higher than iron in the reactivity series. Still, Ayesha observes that iron is reacting faster than aluminium because aluminium when exposed to air, forms a layer of aluminium oxide on its surface which makes it less reactive. Whereas iron does not form any such oxide layer and reacts without any hindrance. So, initially iron reacts more vigorously than aluminium but after some time, aluminium oxide layer is removed and aluminum starts reacting more vigorously as it is more reactive than iron. 



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