Class 10 Science · Chapter 3

3. Metals and Non-metals

Complete solutions to the intext questions and exercises. CBSE Class 10 Science (NCERT).

Intext Questions (Page 40)

1Give an example of a metal which (i) is a liquid at room temperature. (ii) can be easily cut with a knife. (iii) is the best conductor of heat. (iv) is a poor conductor of heat.

(i) Liquid at room temperature: Mercury

(ii) Can be easily cut with a knife: Sodium (also potassium and lithium)

(iii) Best conductor of heat: Silver

(iv) Poor conductor of heat: Lead (also mercury)

2Explain the meanings of malleable and ductile.

Malleable: Metals that can be beaten into thin sheets are called malleable. Gold and silver are the most malleable metals.

Ductile: Metals that can be drawn into thin wires are called ductile. Gold is the most ductile metal.

Intext Questions (Page 46)

1Why is sodium kept immersed in kerosene oil?

Sodium reacts very vigorously with oxygen and moisture. If kept in the open, it can catch fire. Sodium does not react with kerosene. So it is kept immersed in kerosene oil to prevent accidental fires.

2Write equations for the reactions of (i) iron with steam (ii) calcium and potassium with water

(i) Iron with steam

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

(ii) Calcium and potassium with water

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) + heat energy

3Samples of four metals A, B, C and D were taken and added to the following solution one by one. Use the results in the table to answer: (i) Which is the most reactive metal? (ii) What would you observe if B is added to a solution of Copper(II) sulphate? (iii) Arrange the metals A, B, C and D in the order of decreasing reactivity.

Metal	Iron(II) sulphate	Copper(II) sulphate	Zinc sulphate	Silver nitrate
A	No reaction	Displacement	–	–
B	Displacement	–	No reaction	–
C	No reaction	No reaction	No reaction	Displacement
D	No reaction	No reaction	No reaction	No reaction

(i) Most reactive metal

Answer: B

B displaces iron from iron(II) sulphate. So it is more reactive than iron. No other metal does this.

(ii) B added to copper(II) sulphate solution

B is more reactive than iron, so it is also more reactive than copper. It will displace copper. The blue colour of the solution will fade. A reddish-brown layer of copper will be deposited on B.

(iii) Decreasing order of reactivity

Answer: B > A > C > D

4Which gas is produced when dilute hydrochloric acid is added to a reactive metal? Write the chemical reaction when iron reacts with dilute H₂SO₄.

Hydrogen gas is produced.

Fe(s) + H₂SO₄(aq) → FeSO₄(aq) + H₂(g)

5What would you observe when zinc is added to a solution of iron(II) sulphate? Write the chemical reaction that takes place.

Zinc is more reactive than iron. So it displaces iron. The pale green colour of the solution fades and a grey layer of iron is deposited on zinc.

Zn(s) + FeSO₄(aq) → ZnSO₄(aq) + Fe(s)

Intext Questions (Page 49)

1(i) Write the electron-dot structures for sodium, oxygen and magnesium. (ii) Show the formation of Na₂O and MgO by the transfer of electrons. (iii) What are the ions present in these compounds?

(i) Electron-dot structures

Sodium (2, 8, 1) has 1 valence electron. Oxygen (2, 6) has 6. Magnesium (2, 8, 2) has 2.

Electron-dot structures of sodium, oxygen and magnesium.

(ii) Formation of Na₂O and MgO

Formation of sodium oxide: each sodium atom gives one electron to oxygen.

Formation of magnesium oxide: magnesium gives two electrons to oxygen.

(iii) Ions present

Na₂O: sodium ions (Na⁺) and oxide ions (O²⁻).

MgO: magnesium ions (Mg²⁺) and oxide ions (O²⁻).

2Why do ionic compounds have high melting points?

In ionic compounds, there are strong forces of attraction between the oppositely charged ions. A large amount of energy is needed to break these forces. So ionic compounds have high melting points.

Intext Questions (Page 53)

1Define the following terms. (i) Mineral (ii) Ore (iii) Gangue

(i) Mineral: The elements or compounds which occur naturally in the earth’s crust are called minerals.

(ii) Ore: Minerals from which a metal can be profitably extracted are called ores.

(iii) Gangue: Ores mined from the earth contain large amounts of impurities such as soil and sand. These impurities are called gangue.

2Name two metals which are found in nature in the free state.

Answer: Gold and platinum

Silver and copper are also sometimes found in the free state.

3What chemical process is used for obtaining a metal from its oxide?

Answer: Reduction

The metal oxide is reduced to metal using a reducing agent like carbon (coke).

ZnO(s) + C(s) → Zn(s) + CO(g)

Intext Questions (Page 55)

1Metallic oxides of zinc, magnesium and copper were heated with the following metals – zinc, magnesium and copper. In which cases will you find displacement reactions taking place?

A more reactive metal displaces a less reactive metal from its oxide. The reactivity order is Mg > Zn > Cu.

Metal oxide	Zinc	Magnesium	Copper
Zinc oxide	No reaction	Displacement	No reaction
Magnesium oxide	No reaction	No reaction	No reaction
Copper oxide	Displacement	Displacement	No reaction

2Which metals do not corrode easily?

Less reactive metals like gold, platinum and silver do not corrode easily.

3What are alloys?

An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. Examples: brass (copper and zinc), bronze (copper and tin), and steel (iron and carbon).

Exercises

1Which of the following pairs will give displacement reactions? (a) NaCl solution and copper metal (b) MgCl₂ solution and aluminium metal (c) FeSO₄ solution and silver metal (d) AgNO₃ solution and copper metal.

Answer: (d) AgNO₃ solution and copper metal.

Copper is more reactive than silver. So it displaces silver from silver nitrate.

2Which of the following methods is suitable for preventing an iron frying pan from rusting? (a) Applying grease (b) Applying paint (c) Applying a coating of zinc (d) All of the above.

Answer: (c) Applying a coating of zinc

A frying pan is heated and washed again and again. Grease and paint would spoil the food or peel off. So galvanising (zinc coating) is suitable.

3An 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 oxide is an ionic compound. It has a high melting point and dissolves in water.

4Food cans are coated with tin and not with zinc because (a) zinc is costlier than tin. (b) zinc has a higher melting point than tin. (c) zinc is more reactive than tin. (d) zinc is less reactive than tin.

Answer: (c) zinc is more reactive than tin.

Zinc can react with acids in the food. Tin is less reactive, so it is safe.

5You are given a hammer, a battery, a bulb, wires and a switch. (a) How could you use them to distinguish between samples of metals and non-metals? (b) Assess the usefulness of these tests in distinguishing between metals and non-metals.

(a) Using the given items

Hammer test: Beat each sample with the hammer. Metals are malleable, so they become flat sheets. Non-metals are brittle, so they break into pieces.

Conductivity test: Make a circuit with the battery, bulb, switch and wires. Place each sample in the gap of the circuit and switch on. If the bulb glows, the sample is a metal. If it does not glow, it is a non-metal.

(b) Usefulness of these tests

These tests are useful because most metals are malleable and good conductors. Most non-metals are brittle and poor conductors.

But they are not fully reliable. Graphite is a non-metal, but it conducts electricity. So we should use more than one test.

6What are amphoteric oxides? Give two examples of amphoteric oxides.

Metal oxides which react with both acids and bases to produce salts and water are called amphoteric oxides.

Examples: aluminium oxide (Al₂O₃) and zinc oxide (ZnO).

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O

7Name two metals which will displace hydrogen from dilute acids, and two metals which will not.

Metals which displace hydrogen from dilute acids: zinc and magnesium (also iron, aluminium).

Metals which do not displace hydrogen: copper and silver (also gold).

8In the electrolytic refining of a metal M, what would you take as the anode, the cathode and the electrolyte?

Anode: a thick block of impure metal M.

Cathode: a thin strip of pure metal M.

Electrolyte: a solution of a soluble salt of metal M.

9Pratyush took sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube over it. (a) What will be the action of gas on (i) dry litmus paper? (ii) moist litmus paper? (b) Write a balanced chemical equation for the reaction taking place.

The gas evolved is sulphur dioxide (SO₂).

(a)(i) Dry litmus paper

There is no effect on dry litmus paper.

(a)(ii) Moist litmus paper

Moist blue litmus paper turns red. Sulphur dioxide forms sulphurous acid with water.

SO₂(g) + H₂O(l) → H₂SO₃(aq)

(b) Balanced equation

S(s) + O₂(g) → SO₂(g)

10State two ways to prevent the rusting of iron.

Painting, oiling or greasing the iron articles.
Galvanising, i.e. coating iron with a thin layer of zinc.

11What type of oxides are formed when non-metals combine with oxygen?

Non-metals form acidic oxides (for example SO₂, CO₂) or neutral oxides (for example CO, H₂O).

12Give reasons

(a) Platinum, gold and silver are used to make jewellery.

They are lustrous, malleable and ductile. They are least reactive, so they do not corrode easily.

(b) Sodium, potassium and lithium are stored under oil.

These metals are highly reactive. They react vigorously with oxygen and moisture and may catch fire. So they are stored under oil.

Aluminium forms a thin layer of aluminium oxide on its surface. This layer protects it from further reaction. Aluminium is also light and a good conductor of heat.

(d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction.

It is easier to obtain a metal from its oxide than from its carbonate or sulphide. So these ores are first changed into oxides by calcination or roasting.

13You must have seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why these sour substances are effective in cleaning the vessels.

Copper reacts with moist carbon dioxide in air and forms a green layer of basic copper carbonate. This layer is basic in nature.

Lemon and tamarind juice contain acids. These acids react with the basic layer and dissolve it. So the vessel becomes shiny again.

14Differentiate between metal and non-metal on the basis of their chemical properties.

Metals	Non-metals
Metals form basic oxides. Some form amphoteric oxides.	Non-metals form acidic or neutral oxides.
Many metals displace hydrogen from dilute acids.	Non-metals do not displace hydrogen from dilute acids.
Metals lose electrons and form positive ions.	Non-metals gain electrons and form negative ions.
Metals form ionic chlorides.	Non-metals form covalent chlorides.
Reactive metals react with water to give hydrogen.	Non-metals do not react with water to give hydrogen.

15A man went door to door posing as a goldsmith. He dipped a set of gold bangles in a particular solution. The bangles sparkled like new but their weight was reduced drastically. Can you play the detective to find out the nature of the solution he had used?

He used aqua regia. It is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3 : 1.

Aqua regia can dissolve gold. When the bangles were dipped in it, the outer layer of gold dissolved. So they looked shiny, but their weight decreased.

16Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).

Copper does not react with cold water, hot water or steam. Iron in steel reacts with steam and also rusts. Copper is also a better conductor of heat. So copper is used for hot water tanks.

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