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

NCERT Class 10 Science — Metals and Non-metals, the five in-text QUESTIONS boxes on pages 40, 46, 49, 53 and 55. All 15 questions solved.

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

The five boxes follow the chapter’s argument in order: the physical properties of metals, then their reactions with water and acids, then the ionic bond that explains those reactions, then how metals occur and are extracted, and finally corrosion and alloys. Three things carry all fifteen questions:

• The reactivity series — a more reactive metal displaces a less reactive one from its salt or its oxide, and only metals above hydrogen displace it from a dilute acid.
• Metals lose electrons, non-metals gain them, and the ionic compound that results is held together by strong electrostatic attraction.
• Mineral, ore, gangue — and the extraction sequence that follows, in which an ore is first turned into an oxide and the oxide then reduced.

Key insight. Where a metal sits in the reactivity series answers almost everything here: why sodium is kept under kerosene, which of four unknown metals is most reactive, which oxide gets displaced by which metal, and which metals never corrode. The one table question on page 46 looks like a puzzle but is just the series being reconstructed from evidence — each “displacement” or “no reaction” is one inequality, and five of them pin the order down completely.

Questions, page 40. The box that closes Section 3.1, on the physical properties of metals and non-metals.

Page 40, Question 1

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.

Solution. Each part asks for an exception to the usual picture of a metal, which is why the question is worth answering carefully.

(i) Metals are solids at room temperature — with one exception. Mercury is a liquid, which is why it was used in thermometers.

(ii) Metals are normally hard, but the alkali metals are not. Sodium is soft enough to be cut with a knife, and so are potassium and lithium.

(iii) Metals conduct heat well, and the best of all is silver. Copper comes next, which is why cooking pans are made of copper and aluminium rather than silver — silver would be better, and far too expensive.

(iv) Some metals conduct heat poorly all the same. Lead is the standard example, and mercury is another.

Page 40, Question 2

Explain the meanings of malleable and ductile.

Solution. Both words describe how a metal behaves when it is worked, and they differ in the shape produced.

Malleable means the metal can be beaten into thin sheets without breaking. Gold and silver are the most malleable; aluminium and copper are malleable enough to be rolled into the foil used for packing food.

Ductile means the metal can be drawn into thin wires. Gold is the most ductile — a gram of it can be drawn into a wire about two kilometres long — and copper’s ductility is why it is used for electrical wiring.

Non-metals are neither: they are brittle and break into pieces when hammered.

Questions, page 46. The box that closes Section 3.2, on the chemical properties of metals — their reactions with oxygen, water, acids and salt solutions.

Page 46, Question 1

Why is sodium kept immersed in kerosene oil?

Solution. Sodium is near the top of the reactivity series, and its reactions with the ordinary contents of air are violent.

It reacts with the oxygen of the air so readily that it cannot be kept exposed, and it reacts with moisture so vigorously that enough heat is produced to set the hydrogen released on fire:

$$2\text{Na(s)} + 2\text{H}_2\text{O(l)} \longrightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)} + \text{heat}$$

Kerosene oil does not react with sodium and does not dissolve in water, so a layer of it seals the metal away from both the air and any moisture. The metal can then be stored and handled safely. Potassium and lithium are stored the same way.

Page 46, Question 2

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

Solution. (i) Iron with steam. Iron is not reactive enough to attack cold water, but it does react with steam, giving iron(II,III) oxide and hydrogen:

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

(ii) Calcium with water. Calcium is higher in the series and reacts with cold water, though less violently than sodium — the bubbles of hydrogen stick to its surface and make it float:

$$\text{Ca(s)} + 2\text{H}_2\text{O(l)} \longrightarrow \text{Ca(OH)}_2\text{(aq)} + \text{H}_2\text{(g)}$$

Potassium with water. Potassium is higher still, and the reaction is so violent that the hydrogen catches fire immediately:

$$2\text{K(s)} + 2\text{H}_2\text{O(l)} \longrightarrow 2\text{KOH(aq)} + \text{H}_2\text{(g)} + \text{heat}$$

The three together show the pattern: the higher the metal in the reactivity series, the more vigorous its reaction with water.

Page 46, Question 3

Samples of four metals A, B, C and D were taken and added to the following solutions one by one. Use 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.

Solution. Every entry in the table is one inequality, and putting them together pins down the order. A metal displaces another only if it is more reactive.

From A: no reaction with iron(II) sulphate, but displacement with copper(II) sulphate. So A is below iron and above copper: $\text{Fe} > \text{A} > \text{Cu}$.

From B: displacement with iron(II) sulphate, no reaction with zinc sulphate. So B is above iron and below zinc: $\text{Zn} > \text{B} > \text{Fe}$.

From C: no reaction with iron, copper or zinc sulphate, but displacement with silver nitrate. So C is below copper and above silver: $\text{Cu} > \text{C} > \text{Ag}$.

From D: no reaction with anything at all, not even silver nitrate. So D is below silver — the least reactive of the four.

Chaining these: $\text{B} > \text{A} > \text{C} > \text{D}$.

(i) The most reactive metal is B, the only one above iron.

(ii) B is more reactive than iron, and iron is more reactive than copper, so B is more reactive than copper. Adding B to copper(II) sulphate therefore gives a displacement reaction: the blue colour of the solution fades as copper leaves it, and a reddish-brown deposit of copper forms on the surface of B.

(iii) In decreasing order of reactivity: B > A > C > D.

Page 46, Question 4

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

Solution. A metal above hydrogen in the reactivity series displaces hydrogen from a dilute acid, so the gas produced is hydrogen. It can be identified by bringing a burning splint near it: it burns with a pop.

Iron is above hydrogen, so with dilute sulphuric acid it gives iron(II) sulphate and hydrogen:

$$\text{Fe(s)} + \text{H}_2\text{SO}_4\text{(dil.)} \longrightarrow \text{FeSO}_4\text{(aq)} + \text{H}_2\text{(g)}$$

Page 46, Question 5

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

Solution. Zinc lies above iron in the reactivity series, so zinc displaces iron from its salt solution:

$$\text{Zn(s)} + \text{FeSO}_4\text{(aq)} \longrightarrow \text{ZnSO}_4\text{(aq)} + \text{Fe(s)}$$

What you would see follows from the colours. Iron(II) sulphate solution is light green; zinc sulphate solution is colourless. So the green colour of the solution fades as the iron leaves it, and a greyish deposit of iron appears on the zinc.

Questions, page 49. The box that closes Section 3.3, on how metals and non-metals react — the ionic bond and the properties of ionic compounds.

Page 49, Question 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?

Solution. (i) The electron-dot structure shows only the valence electrons — those in the outermost shell.

Sodium has the configuration 2, 8, 1, so one valence electron. Magnesium is 2, 8, 2, so two. Oxygen is 2, 6, so six.

(ii) Each atom moves towards a complete octet. Sodium and magnesium are metals and lose their valence electrons; oxygen is a non-metal and gains two.

For sodium oxide, one oxygen atom needs two electrons and each sodium can give only one, so two sodium atoms are needed. For magnesium oxide, one magnesium supplies both electrons, so the ratio is one to one.

(iii) In sodium oxide the ions are $\text{Na}^+$ and $\text{O}^{2-}$, in the ratio 2 : 1. In magnesium oxide they are $\text{Mg}^{2+}$ and $\text{O}^{2-}$, in the ratio 1 : 1. Every one of them now has the stable electron arrangement of a noble gas.

Page 49, Question 2

Why do ionic compounds have high melting points?

Solution. An ionic compound is not made of separate molecules. It is a rigid three-dimensional lattice in which every positive ion is surrounded by negative ions and every negative ion by positive ones.

Holding that lattice together is the strong electrostatic force of attraction between the oppositely charged ions, acting in every direction. Melting the solid means breaking enough of those attractions for the ions to move past one another — and because the forces are strong and numerous, that takes a great deal of energy.

A large energy requirement means a high temperature, so ionic compounds melt only when strongly heated. Sodium chloride, for instance, melts at 1074 K.

Questions, page 53. The box that follows Section 3.4, on the occurrence of metals and their extraction and refining.

Page 53, Question 1

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

Solution. The three terms narrow down from “what is in the ground” to “what is worth digging up” to “what has to be thrown away”.

(i) Mineral. The elements or compounds that occur naturally in the earth’s crust are called minerals. Every metal compound found in the ground is a mineral, whether or not it is useful.

(ii) Ore. Some minerals contain a high enough percentage of a particular metal that the metal can be extracted from them profitably. Those minerals are called ores. So every ore is a mineral, but not every mineral is an ore — the difference is whether extraction pays.

(iii) Gangue. Ores dug from the earth are mixed with impurities — soil, sand and rocky material. These impurities are called gangue, and they must be removed before the metal is extracted.

Page 53, Question 2

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

Solution. A metal is found uncombined only if it is unreactive enough not to have been attacked by the air, water and other substances around it over geological time. Those are the metals at the bottom of the reactivity series.

Gold and platinum are the standard examples; silver and copper are sometimes found free as well, though usually as compounds.

The reactive metals at the top of the series — sodium, potassium, calcium, magnesium — are never found free, because they would have reacted long ago.

Page 53, Question 3

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

Solution. In an oxide the metal is combined with oxygen; obtaining the free metal means taking that oxygen away, and removing oxygen is by definition reduction.

How the reduction is done depends on where the metal stands in the reactivity series. Oxides of metals in the middle of the series are reduced by heating with carbon:

$$\text{ZnO(s)} + \text{C(s)} \longrightarrow \text{Zn(s)} + \text{CO(g)}$$

Oxides of the highly reactive metals at the top — sodium, magnesium, aluminium — hold their oxygen too tightly for carbon to remove, and are reduced by electrolysis of the molten oxide or chloride instead.

Questions, page 55. The last box, after Section 3.5 on corrosion and its prevention, and on alloys.

Page 55, Question 1

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

Solution. A metal displaces another from its oxide on the same principle as from a salt solution: the free metal must be more reactive than the metal in the compound. Among these three the order is

$$\text{Mg} > \text{Zn} > \text{Cu}$$

Now test each of the nine combinations. A metal never displaces itself, so the three on the diagonal are out at once.

Oxide heated with →	Zinc	Magnesium	Copper
Zinc oxide	—	Displacement	No reaction
Magnesium oxide	No reaction	—	No reaction
Copper oxide	Displacement	Displacement	—

Magnesium is above zinc, so it displaces zinc from zinc oxide. Both magnesium and zinc are above copper, so both displace copper from copper oxide. Magnesium oxide is displaced by nothing here, because magnesium is the most reactive of the three.

$$\text{ZnO} + \text{Mg} \longrightarrow \text{MgO} + \text{Zn}$$

$$\text{CuO} + \text{Zn} \longrightarrow \text{ZnO} + \text{Cu}$$

$$\text{CuO} + \text{Mg} \longrightarrow \text{MgO} + \text{Cu}$$

Page 55, Question 2

Which metals do not corrode easily?

Solution. Corrosion is the attack of a metal by the air, moisture and acids around it, so the metals that resist it are the ones that react least — those at the bottom of the reactivity series.

Gold and platinum are the least reactive of the common metals and do not corrode at all, which is why gold ornaments recovered from ancient sites are still bright. Silver resists corrosion well too, although it does tarnish black slowly with the sulphur in the air.

This is also why these three are the metals used for jewellery and for electrical contacts that must stay clean.

Page 55, Question 3

What are alloys?

Solution. 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 components in it, then letting the mixture cool.

Alloys are made because they have properties the pure metal lacks. Pure iron is soft, but steel — iron with a little carbon, a non-metal — is hard and strong. Stainless steel, iron with chromium and nickel, does not rust at all. Brass is copper and zinc, bronze copper and tin, and solder lead and tin, whose low melting point is what makes it useful for joining wires.

Common mistakes

• Guessing the order in Question 3 of the page-46 box instead of deriving it. Each entry is one inequality: B displaces iron but not zinc, so B sits between them. Writing the four constraints down settles B > A > C > D with no guesswork.
• Saying iron reacts with cold water. It does not — only with steam, and the product is $\text{Fe}_3\text{O}_4$, not $\text{Fe(OH)}_2$. Calcium and potassium react with cold water; iron does not.
• Writing all the valence electrons of the atom in an electron-dot structure. Only the outermost shell is shown — one dot for sodium, two for magnesium, six for oxygen.
• Saying ionic compounds melt at high temperatures “because they are heavy”. The reason is the strength of the electrostatic attraction in the lattice and the energy needed to overcome it.
• Treating every mineral as an ore. An ore is a mineral from which extraction is profitable; the word carries an economic meaning, not only a chemical one.
• Missing a case in the oxide table. There are nine combinations, three of them a metal with its own oxide. Ruling those out first leaves six to test, of which three give displacement.

Practise next

• The end-of-chapter Exercises for this chapter, where Questions 1, 7 and 15 use the same reactivity series on harder cases, and Questions 8 and 12(d) develop the extraction met here.
• Chapter 2, Acids, Bases and Salts, whose in-text questions cover the reaction of metals with acids that this chapter’s Question 4 relies on.