Complete solutions to all 15 in-text questions and all 16 end-of-chapter exercise questions from NCERT Science, Chapter 3 “Metals and Non-metals”, in the English edition’s own wording. Try working out the answer yourself first, then check it against this one.
A In-text Questions
The question boxes printed inside the chapter, after the sections they test, answered first in book order. The end-of-chapter exercises follow below.
Questions after Section 3.1
Q. Give 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.
Answer: (i) Mercury. (ii) Sodium (lithium and potassium can also be cut with a knife). (iii) Silver (copper is almost as good). (iv) Lead (mercury is also a comparatively poor conductor of heat).
Q. Explain the meanings of malleable and ductile.
Answer: Malleable: a metal that can be beaten into thin sheets is malleable; gold and silver are the most malleable metals. Ductile: a metal that can be drawn into thin wires is ductile; gold is the most ductile metal, about 2 km of wire can be drawn from one gram of it.
Questions after Section 3.2
Q. Why is sodium kept immersed in kerosene oil?
Answer: Sodium reacts so vigorously with the oxygen and moisture in air that it catches fire if kept in the open. It is kept immersed in kerosene oil to protect it from air and to prevent accidental fires.
Q. Write equations for the reactions of (i) iron with steam (ii) calcium and potassium with water.
Answer: (i) 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
(ii) Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) + heat energy
Q. Samples of four metals A, B, C and D were taken and added to the following solution one by one. The results obtained have been tabulated as follows.
| 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 |
Use the Table above to answer the following questions about metals A, B, C and D. (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.
Answer: A metal displaces another metal from its salt solution only if it is more reactive than that metal. So: A is more reactive than copper but less reactive than iron; B is more reactive than iron but less reactive than zinc; C is more reactive than silver but less reactive than copper; D is less reactive than silver.
(i) B is the most reactive metal.
(ii) B is more reactive than iron, which is more reactive than copper, so B displaces copper from copper(II) sulphate: the blue colour of the solution fades and a reddish-brown coating of copper forms on B.
(iii) B > A > C > D.
Q. Which gas is produced when dilute hydrochloric acid is added to a reactive metal? Write the chemical reaction when iron reacts with dilute H₂SO₄.
Answer: Hydrogen gas is produced.
Fe(s) + H₂SO₄(aq) → FeSO₄(aq) + H₂(g)
Q. What would you observe when zinc is added to a solution of iron(II) sulphate? Write the chemical reaction that takes place.
Answer: Zinc is more reactive than iron, so it displaces iron from the solution. The pale green colour of the iron(II) sulphate solution fades (the solution becomes colourless) and a dark grey deposit of iron forms on the zinc.
Zn(s) + FeSO₄(aq) → ZnSO₄(aq) + Fe(s)
Questions after Section 3.3
Q. (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?
Answer: (i) Only the valence (outermost) electrons are shown as dots: sodium (2,8,1) has 1 dot, Na•; oxygen (2,6) has 6 dots around O; magnesium (2,8,2) has 2 dots, Mg with two dots (••).
(ii) Na₂O: each of two sodium atoms loses one electron, and the oxygen atom gains both: 2Na → 2Na⁺ + 2e⁻; O + 2e⁻ → O²⁻; so 2Na⁺ + O²⁻ → Na₂O.
MgO: the magnesium atom loses two electrons and the oxygen atom gains them: Mg → Mg²⁺ + 2e⁻; O + 2e⁻ → O²⁻; so Mg²⁺ + O²⁻ → MgO.
(iii) Na₂O contains sodium ions (Na⁺) and oxide ions (O²⁻). MgO contains magnesium ions (Mg²⁺) and oxide ions (O²⁻).
Q. Why do ionic compounds have high melting points?
Answer: Ionic compounds are held together by a strong force of attraction between the oppositely charged ions. A considerable amount of energy is needed to break this strong inter-ionic attraction, so their melting points are high.
Questions after Section 3.4
Q. Define the following terms. (i) Mineral (ii) Ore (iii) Gangue
Answer: (i) Mineral: an element or compound that occurs naturally in the earth’s crust.
(ii) Ore: a mineral that contains a very high percentage of a particular metal, so that the metal can be profitably extracted from it.
(iii) Gangue: the impurities such as soil and sand that come mixed with an ore mined from the earth.
Q. Name two metals which are found in nature in the free state.
Answer: Gold and silver (platinum and copper are also found in the free state).
Q. What chemical process is used for obtaining a metal from its oxide?
Answer: Reduction. For example, zinc oxide is reduced to zinc by heating it with carbon: ZnO(s) + C(s) → Zn(s) + CO(g). Highly reactive metals such as aluminium can also be used as reducing agents.
Questions after Section 3.5
Q. Metallic oxides of zinc, magnesium and copper were heated with the following metals.
| Metal | Zinc | Magnesium | Copper |
|---|---|---|---|
| Zinc oxide | |||
| Magnesium oxide | |||
| Copper oxide |
In which cases will you find displacement reactions taking place?
Answer: A metal displaces another metal from its oxide only if it is more reactive. The order of reactivity is Mg > Zn > Cu. So displacement takes place in three cases:
Zinc oxide heated with magnesium: ZnO + Mg → MgO + Zn
Copper oxide heated with zinc: CuO + Zn → ZnO + Cu
Copper oxide heated with magnesium: CuO + Mg → MgO + Cu
Magnesium oxide does not react with zinc or copper, and no metal reacts with its own oxide.
Q. Which metals do not corrode easily?
Answer: Metals at the bottom of the reactivity series, such as gold and platinum, do not corrode easily. (Silver is also unreactive, but it slowly turns black in air as a coating of silver sulphide forms. Aluminium is protected from further corrosion by its own thin oxide layer.)
Q. What are alloys?
Answer: An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal. It is made by melting the primary metal and dissolving the other elements in it in definite proportions, then cooling it to room temperature. Examples: brass (copper and zinc), bronze (copper and tin), solder (lead and tin).
B End-of-chapter Exercises
1 Which 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 solution: Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag. In the other three pairs, the first metal is less reactive than the second, so no displacement occurs.
2 Which 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: (d) All of the above. Grease, paint and a zinc coating (galvanisation) all keep the iron surface from direct contact with air and water, which is what prevents rusting.
3 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 (CaO), which has a very high melting point (2850 K) and also dissolves in water to form an alkali (calcium hydroxide). Carbon and silicon are non-metals, and iron’s oxide is not soluble in water.
4 Food 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 sits above tin in the reactivity series, meaning it is more reactive. Zinc would react more readily with the acidic content of food, so the less reactive tin is used as the coating for health reasons.
5 You 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.
Answer (a): Strike the sample gently with the hammer: if it flattens into a sheet (malleable), it is a metal; if it breaks into pieces, it is a non-metal. Then connect the sample into a circuit made with the battery, bulb, wires and switch: if the bulb lights up, the sample conducts electricity (usually a metal); if it does not, the sample does not conduct (usually a non-metal).
Answer (b): These tests are not fully reliable, because exceptions exist: graphite (a non-metal) conducts electricity, while sodium and potassium (metals) are so soft they can be cut with a knife instead of needing a hammer. So these tests give a useful first impression, but chemical properties must also be checked for a certain classification.
6 What are amphoteric oxides? Give two examples of amphoteric oxides.
Answer: Metal oxides that react with both acids and bases to give a salt and water are called amphoteric oxides. Examples: aluminium oxide (Al₂O₃) and zinc oxide (ZnO). For instance: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O and Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O.
7 Name two metals which will displace hydrogen from dilute acids, and two metals which will not.
Answer: Metals that displace hydrogen (they lie above hydrogen in the reactivity series): magnesium and zinc. Metals that cannot displace hydrogen (they lie below hydrogen): copper and silver.
8 In the electrolytic refining of a metal M, what would you take as the anode, the cathode and the electrolyte?
Answer: A thick rod of impure metal M would be the anode, a thin strip of pure metal M the cathode, and an aqueous solution of a soluble salt of metal M the electrolyte. On passing current, impure metal dissolves from the anode into the solution, and an equal amount of pure metal deposits on the cathode.
9 Pratyush 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 the gas on (i) dry litmus paper? (ii) moist litmus paper?
(b) Write a balanced chemical equation for the reaction taking place.
Answer (a): (i) The gas has no effect on dry litmus paper, because acidic behaviour needs water to be present. (ii) The gas turns moist litmus paper red, because it combines with the water present to form an acid.
Answer (b): S(s) + O₂(g) → SO₂(g). This is sulphur dioxide gas, which combines with water to form sulphurous acid: SO₂ + H₂O → H₂SO₃, which turns blue litmus red.
10 State two ways to prevent the rusting of iron.
Answer: 1. Applying a coat of paint or oil/grease, to keep the iron surface from direct contact with air and water. 2. Galvanisation: coating the iron surface with a thin layer of zinc.
11 What type of oxides are formed when non-metals combine with oxygen?
Answer: Non-metals combine with oxygen to form acidic or neutral oxides. For example, sulphur dioxide (SO₂) is an acidic oxide.
12 Give reasons
(a) Platinum, gold and silver are used to make jewellery.
(b) Sodium, potassium and lithium are stored under oil.
(c) Aluminium is a highly reactive metal, yet it is used to make utensils for cooking.
(d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction.
Answer (a): Platinum, gold and silver lie at the bottom of the reactivity series, that is, they are very unreactive and have a high lustre. Gold and platinum do not corrode at all, and silver changes only slowly, so jewellery made from them keeps its shine.
Answer (b): These three metals are so reactive that they can react rapidly with the oxygen and moisture in air and catch fire. Keeping them immersed in oil stops them from coming into direct contact with air.
Answer (c): A thin, strong layer of oxide forms on aluminium’s surface, protecting the metal from further oxidation and corrosion, which is why it stays safe for everyday use as cooking utensils.
Answer (d): It is far easier (less energy-intensive) to obtain a metal from its oxide than from a carbonate or a sulphide, so roasting and calcination are used to convert the ore to an oxide first.
13 You must have seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why these sour substances are effective in cleaning the vessels.
Answer: Copper reacts with moist carbon dioxide in air to form a green layer of basic copper carbonate, which makes the vessel look tarnished. The sour acids present in lemon and tamarind juice (citric acid, tartaric acid) react with this basic carbonate layer and dissolve it, bringing back the vessel’s original shiny surface.
14 Differentiate between metal and non-metal on the basis of their chemical properties.
Answer:
| Basis | Metal | Non-metal |
|---|---|---|
| With oxygen | Forms basic oxides | Forms acidic or neutral oxides |
| With water | Some metals react to give hydrogen | Generally do not react |
| With dilute acid | Reacts, displacing hydrogen gas | Does not displace hydrogen |
| Electron transfer | Loses electrons to form cations | Gains electrons to form anions |
15 A man went door to door posing as a goldsmith. He promised to bring back the glitter of old and dull gold ornaments. An unsuspecting lady gave a set of gold bangles to him which he dipped 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?
Answer: Jewellery gold in India is typically 22 carat, meaning a small amount of silver or copper is mixed into the pure gold. The solution the man used was most likely aqua regia (a freshly prepared 3:1 mixture of concentrated hydrochloric acid and concentrated nitric acid), one of the few reagents that can dissolve gold even though neither acid can do so on its own. It dissolved away some of the pure gold itself, not just the silver/copper alloyed into the ornament, which is why the bangles sparkled like new but their weight fell drastically. A strong acid alone, such as nitric acid, cannot dissolve gold at all, so it could not explain what happened to the bangles.
16 Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).
Answer: Copper lies below hydrogen in the reactivity series and does not react with water at all, so it does not corrode even with constant exposure to hot water. Steel is made from iron, which rusts in the presence of both air and water, so it is not suitable for a hot water tank. Copper is also an excellent conductor of heat, which helps heat the water quickly.