NCERT Exemplar Solutions Class 10 Science Metals and Non-metals — Multiple Choice Questions Q1–18

NCERT Exemplar Problems, Class 10 Science — Metals and Non-metals, Multiple Choice Questions 1 to 18. All 18 questions solved.

This unit has the longest multiple-choice band in the whole Class 10 Science exemplar — thirty-six questions — so it is set out here in two halves. This first half covers physical properties, the reactions of metals with water and acids, and the beginning of the occurrence-and-extraction material; Questions 19 to 36 carry on into ionic bonding, alloys, corrosion and refining.

Three ideas decide almost everything in this half:

• The reactivity series. K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au. Where a metal sits relative to hydrogen decides whether it reacts with water and with acids; where two metals sit relative to each other decides which displaces which.
• The difference between cold water, hot water and steam. These are three different tests, not one, and the exemplar separates metals by exactly which of them they respond to.
• Physical properties as a family — lustre, malleability, ductility, sonority, conductivity, high melting point. Metals have them, non-metals generally do not, and the memorable questions are the exceptions.

Key insight. A question that asks which metal does not do something is really asking you to find the lowest one in the reactivity series among the options; a question that asks which one does is asking for the highest. Ordering the four options by reactivity before reading the question again turns most of these into a one-second answer.

Choose the correct answer from the given four options in the following questions:

Question 1

Which of the following property is generally not shown by metals?

Solution. Three of these are in the standard list of metallic properties. Metals conduct electricity because their outer electrons are free to move; they are sonorous, meaning they ring when struck, which is why bells and cymbals are metal; and they are ductile, meaning they can be drawn into wire.

Dullness is the odd one out, and it is the opposite of a metallic property. Metals are lustrous — a freshly cut or polished surface shines, because the free electrons reflect light efficiently. Non-metals are the dull ones, with iodine and graphite as the exceptions.

A metal left in the air may look dull, but that is a layer of oxide or sulphide on the surface, not the metal itself. Scratch or polish it and the shine returns — which is the whole of Question 38 in the previous unit.

Question 2

The ability of metals to be drawn into thin wire is known as

Solution. Four properties, four names, and the pair that gets confused is the first two.

Ductility is being drawn into a wire — a one-dimensional shape. Malleability is being hammered into a sheet — a two-dimensional shape. Gold is the champion of both: one gram can be drawn into two kilometres of wire, or beaten into a sheet a square metre across.

A hook that survives the exam: ductile and duct share a root, and a duct is a long thin tube; malleable comes from the Latin malleus, a hammer, and you hammer things flat.

Sonority is ringing when struck, and conductivity is carrying heat or electricity. Neither has anything to do with shaping.

Question 3

Aluminium is used for making cooking utensils. Which of the following properties of aluminium are responsible for the same?

(i) Good thermal conductivity (ii) Good electrical conductivity (iii) Ductility (iv) High melting point

Solution. The test is not whether aluminium has the property but whether the property is what makes it suitable for cooking. All four statements are true of aluminium; only two are relevant.

(i) Good thermal conductivity is the first requirement of any cooking vessel — heat has to pass quickly from the flame through the metal into the food, or the pan burns at the bottom and stays cold higher up.

(iv) High melting point is the second. A pan sits directly in a flame at several hundred degrees, and a metal that softened or melted there would be useless. Aluminium melts at about $660^\circ\mathrm{C}$, comfortably above any cooking temperature.

(ii) Electrical conductivity is irrelevant to a saucepan — it is why aluminium is used in overhead power cables, a different application entirely. (iii) Ductility is about drawing wire, again a property that matters for cables and not for pans. (Malleability would at least be relevant to shaping the pan; ductility is not.)

Question 4

Which one of the following metals do not react with cold as well as hot water?

Solution. Reaction with water follows the reactivity series exactly, and the syllabus divides metals into three bands:

• React with cold water: potassium, sodium, calcium. Sodium so violently that the hydrogen catches fire; calcium more gently, with bubbles clinging to the metal.
• React with hot water only: magnesium. Cold water barely touches it.
• React with steam only: aluminium, zinc, iron.
• No reaction with water at all: lead, copper, silver, gold.

So sodium and calcium are out on the first band, magnesium on the second. Iron reacts only with steam, and steam is neither cold water nor hot water — it is a third condition. Iron is the answer.

The distinction is worth taking seriously because Question 63 in the long-answer band is built entirely on it: three metals identified by which of the three water tests they respond to.

Question 5

Which of the following oxide(s) of iron would be obtained on prolonged reaction of iron with steam?

Solution. Iron and steam give the mixed oxide $\mathrm{Fe_3O_4}$, magnetic iron oxide, and hydrogen:

$$3\mathrm{Fe}(s) + 4\mathrm{H_2O}(g) \rightarrow \mathrm{Fe_3O_4}(s) + 4\mathrm{H_2}(g)$$

This is the same reaction met in the previous unit’s Questions 3 and 24, and it is worth knowing as a fixed equation because the product is not the one intuition suggests.

$\mathrm{Fe_2O_3}$ is what rust is, and rusting needs both oxygen and moisture together over a long time at ordinary temperature. Steam alone, at high temperature, gives the black $\mathrm{Fe_3O_4}$ instead — and note that the reaction is reversible, which is why the hydrogen has to be swept away for it to continue.

$\mathrm{FeO}$ is a genuine compound but is not the product here. Option (d) is the trap for anyone who half-remembers that iron has two common oxides and hedges.

Question 6

What happens when calcium is treated with water?

(i) It does not react with water (ii) It reacts violently with water (iii) It reacts less violently with water (iv) Bubbles of hydrogen gas formed stick to the surface of calcium

Solution. Note first that (i) and (ii) contradict each other and so do (ii) and (iii), so options (b) and (c) are self-defeating and can go immediately.

Calcium does react with cold water:

$$\mathrm{Ca}(s) + 2\mathrm{H_2O}(l) \rightarrow \mathrm{Ca(OH)_2}(aq) + \mathrm{H_2}(g)$$

so (i) is false and option (a) goes too. But it reacts less violently than sodium or potassium — there is no flame and no explosion, just a steady stream of bubbles. Statement (iii) is the right description.

Statement (iv) is the detail the exemplar wants, and it is a real observation. The hydrogen bubbles stick to the calcium’s surface instead of escaping, and because they do, the lump of calcium becomes buoyant and floats. It is also why the heat evolved is not enough to ignite the hydrogen, as it is with sodium: the reaction is slower and the heat is carried away by the water.

Question 7

Generally metals react with acids to give salt and hydrogen gas. Which of the following acids does not give hydrogen gas on reacting with metals (except Mn and Mg)?

Solution. Dilute sulphuric and hydrochloric acids behave as ordinary acids: a metal above hydrogen displaces the hydrogen and it bubbles off.

Nitric acid is different, and the reason is that it is a strong oxidising agent as well as an acid. Any hydrogen produced is immediately oxidised by the nitric acid to water, so none escapes. What comes off instead is an oxide of nitrogen — $\mathrm{N_2O}$, $\mathrm{NO}$ or $\mathrm{NO_2}$, depending on the concentration.

The bracketed exception is worth understanding rather than memorising. Manganese and magnesium with very dilute nitric acid do give hydrogen, because with those two the displacement is fast enough to outrun the oxidation.

This question and Question 40 of the short-answer band are the same question in two forms, and the answer to both is the single word oxidising.

Question 8

The composition of aqua-regia is

Solution. Every option has the same two acids in the same $3:1$ ratio, so the ratio is not what is being tested — the concentrations are.

Aqua regia is three parts concentrated hydrochloric acid to one part concentrated nitric acid, freshly prepared. Both must be concentrated. A dilute mixture will not dissolve gold, because the mechanism needs a high concentration of both the oxidising nitric acid and the complexing chloride ion working together.

That mechanism is worth a sentence, because it explains why the mixture does what neither acid can do alone. The nitric acid oxidises gold atoms to $\mathrm{Au^{3+}}$; on its own that reaction stops almost at once, because the ions are unstable and revert. The chloride ions from the hydrochloric acid immediately lock those ions up as the stable chloroaurate complex, so the oxidation cannot reverse and keeps going.

Question 9

Which of the following are not ionic compounds?

(i) KCl    (ii) HCl    (iii) $\mathrm{CCl_4}$    (iv) NaCl

Solution. An ionic compound forms when electrons are transferred, which happens between a metal and a non-metal. A covalent compound forms when electrons are shared, which happens between two non-metals. So the test is simply whether the first element in the formula is a metal.

• KCl — potassium is a metal. Ionic.
• HCl — hydrogen is a non-metal, and hydrogen chloride is a covalent molecule. Not ionic.
• $\mathrm{CCl_4}$ — carbon is a non-metal. Not ionic.
• NaCl — sodium is a metal. Ionic.

So (ii) and (iii), option (b).

Hydrogen chloride is the interesting one, and the previous unit’s Questions 27 and 33 depend on it. The molecule is covalent, yet its aqueous solution conducts electricity and behaves as a strong acid — because water pulls the polar molecule apart into ions. Being covalent does not prevent ionisation in water; it only means the compound is not already ionic before the water reaches it.

Question 10

Which one of the following properties is not generally exhibited by ionic compounds?

Solution. Options (b) and (d) are the same claim about two different states, and the whole question is the difference between them.

An ionic compound conducts only when its ions are free to move. In the solid, every ion is locked into a rigid crystal lattice; it can vibrate but it cannot travel, so the solid does not conduct. Melt it, or dissolve it in water, and the lattice breaks apart — the ions become mobile and the substance conducts well.

So (d) is a genuine property of ionic compounds and (b) is not, which makes (b) the answer.

The other two are genuine properties for the same underlying reason. The electrostatic attraction between oppositely charged ions throughout the lattice is strong, so a great deal of energy is needed to break it — hence the high melting and boiling points. And water, being highly polar, can surround and separate the ions, so most ionic compounds dissolve in it.

This is exactly the reasoning asked for in Question 61(c) of the long-answer band.

Question 11

Which of the following metals exist in their native state in nature?

(i) Cu    (ii) Au    (iii) Zn    (iv) Ag

Solution. “Native state” means found as the free metal, uncombined. A metal is found that way only if it is unreactive enough to have resisted combining with oxygen, sulphur or carbon dioxide over geological time — so the answer is the least reactive metals on the list.

Ordering the four by the reactivity series gives Zn > Cu > Ag > Au. The two at the bottom are gold and silver, and those are precisely the metals found native — gold nuggets and native silver are both real mineral specimens.

Zinc is far too reactive and is never found free; it is mined as the sulphide, zinc blende. Copper is the borderline case: it does occur native in small quantities, but the exemplar’s list forces a choice of two, and gold and silver are unambiguously less reactive than copper. Since the question offers no option containing all three, the intended pair is (ii) and (iv).

Question 12

Metals are refined by using different methods. Which of the following metals are refined by electrolytic refining?

(i) Au    (ii) Cu    (iii) Na    (iv) K

Solution. Refining is not the same as extraction, and keeping the two apart is the whole difficulty here. Refining means purifying a metal you have already obtained; extraction means getting it out of its ore in the first place.

Electrolytic refining works on the less reactive metals — copper, gold, silver, zinc, tin, nickel. The impure metal is made the anode, a strip of pure metal the cathode, and a salt of the metal the electrolyte; the metal dissolves off the anode and plates onto the cathode, leaving the impurities behind as anode mud.

Sodium and potassium are at the top of the reactivity series. They are obtained by electrolysis of their molten chlorides — that is Question 18 — but that is extraction, not refining, and it cannot be done from an aqueous solution because water would be reduced in preference to the metal.

So the two refined electrolytically here are gold and copper, option (a).

Question 13

Silver articles become black on prolonged exposure to air. This is due to the formation of

Solution. Silver tarnishes because air contains traces of hydrogen sulphide, and silver reacts with it:

$$2\mathrm{Ag}(s) + \mathrm{H_2S}(g) \rightarrow \mathrm{Ag_2S}(s) + \mathrm{H_2}(g)$$

Silver sulphide is black, and it is the thin black film on a neglected spoon.

The other options fail for good reasons. Silver oxide, $\mathrm{Ag_2O}$, is not formed under ordinary conditions — silver is too low in the reactivity series to combine with atmospheric oxygen, which is exactly why it is used for jewellery. Silver nitride is not formed from air at all; nitrogen is far too unreactive.

Note the contrast with the copper of Question 16 and the iron of rusting. All three are corrosion, but each metal picks a different component of the air — silver takes sulphur, copper takes carbon dioxide and moisture, iron takes oxygen and moisture — and each gives a differently coloured product.

Question 14

Galvanisation is a method of protecting iron from rusting by coating with a thin layer of

Solution. Galvanisation is coating iron or steel with zinc, usually by dipping it in the molten metal.

The coating protects in two ways, and the second is the clever one. Physically it keeps air and moisture off the iron. Chemically, zinc is more reactive than iron, so even if the coating is scratched and the iron exposed, it is the zinc that corrodes preferentially — the iron underneath is protected as long as any zinc remains nearby. This is called sacrificial protection, and it is why a scratched galvanised bucket does not rust at the scratch.

Option (a) is a trap on the name: galvanisation is named after Luigi Galvani, not after gallium. Aluminium would be chemically plausible, since it is more reactive than iron and forms a protective oxide, but it is not what the process uses. Silver is less reactive than iron, so a scratch in a silver coating would make the rusting worse, not better.

Question 15

Stainless steel is very useful material for our life. In stainless steel, iron is mixed with

Solution. Stainless steel is iron alloyed with nickel and chromium. It is the standard composition to know, along with these others the chapter uses:

Alloy	Composition
Stainless steel	Iron + nickel + chromium
Brass	Copper + zinc
Bronze	Copper + tin
Solder	Lead + tin
Amalgam	Any metal + mercury

Chromium is what makes it stainless: it forms a thin, invisible, self-repairing layer of chromium oxide on the surface that stops air and water reaching the iron. Nickel adds toughness and makes the alloy easier to work.

The wider point is the one Question 22 asks directly — alloying changes properties. Pure iron is soft and rusts readily; adding these two makes it hard and rust-resistant, which is the whole reason for the alloy.

Question 16

If copper is kept open in air, it slowly loses its shining brown surface and gains a green coating. It is due to the formation of

Solution. The green coating on old copper — the patina on roofs and statues — is basic copper carbonate, formed from copper, the moisture of the air and its carbon dioxide:

$$2\mathrm{Cu} + \mathrm{H_2O} + \mathrm{CO_2} + \mathrm{O_2} \rightarrow \mathrm{Cu(OH)_2 \cdot CuCO_3}$$

Of the four options only $\mathrm{CuCO_3}$ names that carbonate, so it is the answer.

The others are eliminated by asking where the other element would have come from. Sulphate would need sulphur, and while polluted air does contain some sulphur dioxide, it is not the normal cause. Nitrate would need nitric acid. Copper oxide, CuO, is real but it is black, not green — it is what forms when copper is heated strongly in air, the reaction of Question 16 in the first unit and Question 56 later in this one. Colour alone rules it out here.

Question 17

Generally, metals are solid in nature. Which one of the following metals is found in liquid state at room temperature?

Solution. Mercury is the only metal that is liquid at ordinary room temperature; it freezes at about $-39^\circ\mathrm{C}$. That is why it is used in thermometers and barometers, and why its alloys with other metals are given their own name, amalgams — the subject of Question 28.

Sodium is soft enough to cut with a knife but is a solid, melting at $98^\circ\mathrm{C}$. Iron and chromium melt well above $1500^\circ\mathrm{C}$.

Two related facts are worth carrying, because Question 52 asks for them directly. Bromine is the only non-metal that is liquid at room temperature. And caesium and gallium melt just above room temperature — gallium at $30^\circ\mathrm{C}$, low enough to melt in the palm of the hand — so they are liquid on a warm day but not at the standard room temperature that makes mercury unique.

Question 18

Which of the following metals are obtained by electrolysis of their chlorides in molten state?

(i) Na    (ii) Ca    (iii) Fe    (iv) Cu

Solution. How a metal is extracted depends on where it sits in the reactivity series, and the chapter divides it into three bands:

• Top of the series (K, Na, Ca, Mg, Al) — electrolysis of the molten salt or oxide. These metals hold their compounds so strongly that no chemical reducing agent is powerful enough.
• Middle (Zn, Fe, Pb, Cu) — roast or calcine the ore to the oxide, then reduce with carbon.
• Bottom (Hg, Ag, Au) — often found native, or obtained by simply heating the ore.

Sodium and calcium are in the top band, so they are obtained by electrolysing their molten chlorides. Iron and copper are in the middle band and are obtained by carbon reduction, which is much cheaper.

Two details are worth noting. The chloride must be molten, not dissolved — electrolysing an aqueous solution would discharge hydrogen from the water instead of the metal, which is exactly what happens in the chlor-alkali process. And the extraction of Question 12 is a different operation from the refining of Question 12: sodium is extracted electrolytically but not refined electrolytically.

Common mistakes

• Confusing ductility with malleability in Questions 2 and 3. Ductile means drawn into wire; malleable means hammered into sheets. Both appear as options in the same question precisely because they are confused.
• Treating “reacts with water” as one test, in Questions 4 and 6. Cold water, hot water and steam separate the metals into three different bands, and a question naming one of them means only that one.
• Expecting hydrogen from nitric acid, in Question 7. Nitric acid is an oxidising acid; it oxidises the hydrogen to water as fast as it forms, so an oxide of nitrogen comes off instead.
• Missing that both acids in aqua regia must be concentrated, in Question 8. Every option has the same $3:1$ ratio, so the ratio is not the question; the concentrations are.
• Saying ionic compounds conduct in the solid state, in Question 10. They conduct only when molten or dissolved, because only then can the ions move. It is the single most common error in this chapter.
• Confusing extraction with refining, in Questions 12 and 18. Sodium is extracted by electrolysis but is not refined by electrolysis; copper is both. The two words name different stages.
• Choosing CuO for the green coating in Question 16. Copper oxide is black. The green patina is basic copper carbonate, and colour alone decides it.
• Forgetting that rust is $\mathrm{Fe_2O_3}$ but the steam product is $\mathrm{Fe_3O_4}$, in Question 5. Rusting needs oxygen and moisture at ordinary temperature; steam at high temperature gives the different, magnetic oxide.

Practise next

• Exemplar Multiple Choice Questions Q19–36 — the second half of this band, carrying on into ionic bonding, alloys and the electrolytic refining diagram.
• Exemplar Short and Long Answer Questions — where the reactivity series has to be used to write equations rather than pick options.
• NCERT textbook Chapter 3 — Metals and Non-metals for the end-of-chapter exercise on properties, the activity series and extraction.
• Textbook activities for the experiments behind Questions 4, 6 and 16 — the metal-and-water tests are done there, and seeing them is what fixes the three bands in memory.