NCERT Solutions Class 10 Science Chapter 3 — Metals and Non-metals

NCERT Class 10 Science — Metals and Non-metals, Chapter 3 Exercises. All 16 questions solved.

Also here. Solutions to the in-text questions for this chapter and the activities for this chapter are on their own pages.

The exercise is built on one idea used over and over — the reactivity series — and three things that follow from it. Questions 1, 4, 7 and 15 are the series applied directly. Questions 6, 9, 11 and 14 are about what metals and non-metals do with oxygen. Questions 2, 10, 13 and 16 are about corrosion and how to stop it, and Questions 8 and 12(d) are about getting a metal out of its ore in the first place.

• The reactivity series — potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, (hydrogen), copper, silver, gold. A metal displaces any metal below it from a salt solution, and only those above hydrogen displace it from a dilute acid.
• Metal oxides are basic, non-metal oxides are acidic or neutral — and a few metal oxides, the amphoteric ones, behave both ways.
• Extraction runs downhill through the series: the more reactive the metal, the harder it is to get out of its ore, and the more energetic the method must be.

Key insight. Almost every “give reasons” question here is answered by locating the metal in the reactivity series and then saying what that position implies. Sodium is stored under oil because it is near the top; gold is used for jewellery because it is at the bottom; aluminium is high in the series yet safe for cooking pans, and the exception is worth knowing — its oxide layer is protective, which is the one thing that lets a reactive metal behave like an unreactive one.

Question 1

Which of the following pairs will give displacement reactions?

Solution. A displacement reaction happens only when the free metal is more reactive than the metal in the salt. So check each pair against the reactivity series.

(a) Sodium is far above copper, so copper cannot displace it. No reaction.

(b) Magnesium is above aluminium, so aluminium cannot displace it. No reaction.

(c) Iron is above silver, so silver cannot displace it. No reaction.

(d) Copper is above silver, so copper can displace silver from silver nitrate:

$$\text{Cu(s)} + 2\text{AgNO}_3\text{(aq)} \longrightarrow \text{Cu(NO}_3)_2\text{(aq)} + 2\text{Ag(s)}$$

The colourless solution turns blue and grey silver is deposited on the copper.

Question 2

Which of the following methods is suitable for preventing an iron frying pan from rusting?

Solution. All three methods do prevent rusting in general — but the question is about a frying pan, which is heated strongly and used for food. That rules two of them out.

Grease would melt and burn off the moment the pan was heated, and would contaminate the food.

Paint would likewise burn and peel at cooking temperatures, and paint is not safe in contact with food.

A coating of zinc — galvanisation — survives heating and stays put. Zinc is more reactive than iron, so it also protects the iron even if the coating is scratched. This is the only method suited to a frying pan.

Question 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

Solution. Two clues, and both point to a reactive metal.

A high melting point for the oxide suggests an ionic compound, and ionic oxides are formed by metals. Solubility in water narrows it much further — most metal oxides are insoluble; the ones that dissolve belong to the highly reactive metals, and they give alkalis when they do.

Carbon and silicon are non-metals: carbon dioxide is a gas, and silicon dioxide, though high-melting, is insoluble in water. Iron oxide is insoluble too.

Calcium fits both. Calcium oxide has a very high melting point and dissolves in water to form calcium hydroxide — the quick lime and slaked lime of Chapter 1.

$$\text{CaO(s)} + \text{H}_2\text{O(l)} \longrightarrow \text{Ca(OH)}_2\text{(aq)}$$

Question 4

Food cans are coated with tin and not with zinc because

Solution. The coating on a food can has to be unreactive, because it is in constant contact with food, and food is often acidic.

Zinc lies above tin in the reactivity series, so it is the more reactive of the two. It would react with the acids in the food, dissolving into it and making it unsafe to eat.

Tin is much less reactive, so it does not attack the food or dissolve into it. That is why cans are tin-plated rather than galvanised, even though galvanising is the usual way of protecting iron elsewhere.

Options (a) and (b) are irrelevant to safety, and (d) is simply false.

Question 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.

Solution. (a) The tools offer two distinct physical tests.

The hammer — a test of malleability. Beat each sample gently. A metal is malleable: it flattens into a sheet without breaking. A non-metal is brittle in the solid state: it shatters into pieces.

The circuit — a test of electrical conductivity. Connect the battery, bulb and switch in a circuit with a gap, and clamp the sample across the gap. If the bulb glows, the sample conducts electricity and is a metal. If it does not glow, the sample is a non-metal.

(b) The tests are useful as a first indication and both usually work, since most metals are malleable conductors and most non-metals are brittle insulators. But neither is conclusive, and the exceptions are exactly what a good answer should name:

• Graphite is a non-metal that conducts electricity well, so the bulb would glow for it.
• Sodium and potassium are metals so soft they can be cut with a knife — hammering tells you little.
• Diamond is a non-metal that is the hardest known substance.
• Mercury is a metal that is liquid at room temperature, so it can be neither hammered nor held across a gap.

So the two tests together give a good first answer, but a chemical test — whether the element’s oxide is basic or acidic — is what settles it.

Question 6

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

Solution. Metal oxides are normally basic and non-metal oxides acidic. A few metal oxides sit between the two: they react with both acids and bases to give a salt and water. These are called amphoteric oxides.

Aluminium oxide is one:

$$\text{Al}_2\text{O}_3 + 6\text{HCl} \longrightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$$

$$\text{Al}_2\text{O}_3 + 2\text{NaOH} \longrightarrow 2\text{NaAlO}_2 + \text{H}_2\text{O}$$

Zinc oxide is the other:

$$\text{ZnO} + 2\text{HCl} \longrightarrow \text{ZnCl}_2 + \text{H}_2\text{O}$$

$$\text{ZnO} + 2\text{NaOH} \longrightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2\text{O}$$

In the first reaction of each pair the oxide is behaving as a base; in the second it is behaving as an acid. That double behaviour is what “amphoteric” means.

Question 7

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

Solution. A metal displaces hydrogen from a dilute acid only if it lies above hydrogen in the reactivity series — that is what the position of hydrogen in the series is there to tell you.

Above hydrogen, so they will displace it: magnesium and zinc (iron and aluminium would do as well).

$$\text{Mg} + 2\text{HCl} \longrightarrow \text{MgCl}_2 + \text{H}_2$$

Below hydrogen, so they will not: copper and silver (gold and mercury would do as well). They cannot give up electrons to the hydrogen ions of the acid, so no hydrogen is released.

Question 8

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

Solution. Electrolytic refining works by dissolving the impure metal at one electrode and depositing it, pure, at the other. So each part of the cell has a specific job.

Anode — a thick block of the impure metal M. Connected to the positive terminal, it dissolves into the electrolyte as the current flows.

Cathode — a thin strip of pure metal M. Connected to the negative terminal, it is where pure metal is deposited from the solution.

Electrolyte — a solution of a soluble salt of M, usually acidified. It carries the metal ions from the anode across to the cathode.

The impurities do not dissolve. The more reactive ones go into solution and stay there; the less reactive ones fall to the bottom of the cell as anode mud, from which gold and silver are often recovered.

Question 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 gas on (i) dry litmus paper? (ii) moist litmus paper?

(b) Write a balanced chemical equation for the reaction taking place.

Solution. Sulphur is a non-metal, and heating it in air burns it to sulphur dioxide, a non-metal oxide. Non-metal oxides are acidic — but, as Chapter 2 showed, acidic behaviour needs water.

(a)(i) Dry litmus paper: no change. With no water present, the sulphur dioxide cannot form an acid, so there are no hydrogen ions and the litmus keeps its colour.

(a)(ii) Moist litmus paper: blue litmus turns red. On damp paper the gas dissolves and reacts with the water to give sulphurous acid, which supplies the hydrogen ions:

$$\text{SO}_2\text{(g)} + \text{H}_2\text{O(l)} \longrightarrow \text{H}_2\text{SO}_3\text{(aq)}$$

(b) The reaction on heating is

$$\text{S(s)} + \text{O}_2\text{(g)} \xrightarrow{\ \text{heat}\ } \text{SO}_2\text{(g)}$$

Question 10

State two ways to prevent the rusting of iron.

Solution. Rusting needs air and moisture together at the iron’s surface, so every method either keeps them out or puts something else in the way.

1. Coating the surface — painting, oiling, greasing or applying a layer of another metal by electroplating. The coat is a physical barrier between the iron and the damp air.

2. Galvanisation — coating the iron with a layer of zinc. This is better than a simple barrier, because zinc is more reactive than iron, so even if the coating is scratched the zinc corrodes in preference and the iron beneath is still protected.

(Alloying is a third: mixing iron with chromium and nickel gives stainless steel, which does not rust at all.)

Question 11

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

Solution. Non-metals form oxides that are acidic or, in a few cases, neutral.

Most are acidic: they dissolve in water to give acids, and they react with bases to give a salt and water. Sulphur dioxide gives sulphurous acid, and carbon dioxide gives carbonic acid:

$$\text{SO}_2 + \text{H}_2\text{O} \longrightarrow \text{H}_2\text{SO}_3 \qquad \text{CO}_2 + \text{H}_2\text{O} \longrightarrow \text{H}_2\text{CO}_3$$

A few are neutral — neither acidic nor basic. Water ($\text{H}_2\text{O}$), carbon monoxide (CO) and nitrous oxide ($\text{N}_2\text{O}$) are the standard examples.

This is the opposite of what metals do: metal oxides are basic.

Question 12

Give reasons

Solution. Each of these is the reactivity series being read in a different direction.

(a) Platinum, gold and silver are used to make jewellery. They are at the very bottom of the reactivity series, so they are highly unreactive — they do not corrode or tarnish, and jewellery keeps its shine for a lifetime. They are also lustrous, and malleable and ductile, so they can be beaten and drawn into intricate designs.

(b) Sodium, potassium and lithium are stored under oil. They are at the very top, so they are extremely reactive. Exposed to air they react at once with oxygen and with moisture — so vigorously that they catch fire. Kerosene oil keeps air and water away from the surface and makes them safe to store.

(c) Aluminium is a highly reactive metal, yet it is used to make utensils for cooking. This is the exception worth knowing. Aluminium does react with the oxygen of the air — but the thin oxide layer that forms is hard, and it sticks tightly to the surface, sealing the metal beneath from any further attack. The metal is therefore corrosion-resistant in practice. It is also a good conductor of heat and has a high melting point, which is what a cooking pan needs.

(d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction. Because it is far easier to reduce an oxide to the metal than to reduce a carbonate or a sulphide directly. So the ore is first roasted (sulphides) or calcined (carbonates) to the oxide, and the oxide is then reduced with carbon or by electrolysis.

Question 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.

Solution. A copper vessel tarnishes because the metal reacts slowly with the moisture and carbon dioxide of the air, forming a dull green coating of basic copper carbonate on the surface.

That coating is basic, which is exactly why an acid removes it. Lemon juice contains citric acid and tamarind contains tartaric acid, and these acids neutralise and dissolve the green layer, leaving the bright copper surface exposed again.

So this is a neutralisation reaction being put to work in the kitchen, and it is the same chemistry as Chapter 2’s antacid.

Question 14

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

Solution. The question asks for chemical properties, so lustre and malleability are not what it wants — it wants how the two behave in reactions.

	Metals	Non-metals
Electrons	Lose electrons, forming positive ions	Gain (or share) electrons, forming negative ions
Oxides	Form basic oxides (a few amphoteric)	Form acidic or neutral oxides
With dilute acids	Those above hydrogen displace hydrogen	Do not displace hydrogen
With hydrogen	Only the most reactive form hydrides	React to form hydrides
Role in a reaction	Act as reducing agents	Act as oxidising agents
With chlorine	Form ionic chlorides	Form covalent chlorides

Question 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?

Solution. Two facts have to be explained together: the bangles came out shining, and they came out lighter. A solution that merely cleaned off tarnish would do the first but not the second. Losing weight means gold itself was removed.

Gold is one of the least reactive metals and is not attacked by any single acid. The one common reagent that dissolves it is aqua regia — a freshly prepared mixture of three parts concentrated hydrochloric acid to one part concentrated nitric acid.

Dipping the bangles in aqua regia dissolved the outer layer of gold. That removed the dull tarnished surface, so the bangles sparkled like new — and the dissolved gold, which the man kept in the solution, is the weight that went missing.

Question 16

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

Solution. The tank holds hot water, so the question is how each metal behaves towards water and steam.

Iron reacts with steam, forming iron oxide and hydrogen:

$$3\text{Fe(s)} + 4\text{H}_2\text{O(g)} \longrightarrow \text{Fe}_3\text{O}_4\text{(s)} + 4\text{H}_2\text{(g)}$$

A steel tank would corrode from the inside, and rust where water and air met the surface, so it would not last.

Copper does not react with water or steam at all. It sits below hydrogen in the reactivity series, so hot water leaves it untouched and the tank has a long life.

Copper is also an excellent conductor of heat, better than steel, so water in a copper tank heats up faster and more evenly — a second reason for the choice.

Common mistakes

• Choosing (d) “all of the above” in Question 2 because grease and paint genuinely do prevent rusting. They do — but not on a frying pan, which is heated and holds food. Read what the object is before choosing the method.
• Saying zinc is used for food cans because it is cheaper in Question 4. Cost is not the issue; reactivity is. Zinc would react with the acids in the food and contaminate it.
• Answering Question 14 with physical properties — lustre, malleability, conductivity. The question says chemical properties: ions formed, nature of the oxides, behaviour with dilute acids.
• Saying the goldsmith used “some acid” in Question 15. No single acid dissolves gold. Naming aqua regia, and the 3 : 1 ratio, is what the question is testing.
• Forgetting that aluminium’s protection is its oxide layer in Question 12(c). Saying “aluminium is unreactive” contradicts the question itself, which states that it is highly reactive.
• Expecting dry sulphur dioxide to redden litmus in Question 9. Without water there are no hydrogen ions, so dry litmus is unchanged — exactly as for dry hydrogen chloride in Chapter 2.

Practise next

• The in-text questions of this chapter, thirteen across five boxes, which test the same reactivity series and the same reactions of metals with water, acids and oxygen in shorter form.
• Chapter 1, Chemical Reactions and Equations, where displacement and the corrosion of metals first appear, and whose Question 17 is this chapter’s copper oxide in miniature.