NCERT Exemplar Solutions Class 10 Science Metals and Non-metals — Multiple Choice Questions Q19–36

NCERT Exemplar Problems, Class 10 Science — Metals and Non-metals, Multiple Choice Questions 19 to 36. All 18 questions solved.

This is the second half of the unit’s multiple-choice band; Questions 1 to 18 cover physical properties and the reactions with water and acids. From here the exercise turns to what happens when metals and non-metals meet — ionic bonding — and then to alloys, corrosion and the electrolytic refining cell.

Two ideas carry this half:

• Ionic bonding and what follows from it. A metal loses electrons, a non-metal gains them, and the resulting lattice of oppositely charged ions is what gives every ionic compound its family of properties: high melting point, solubility in water, conduction when molten but not when solid.
• The electrode rules. In an electrolytic cell the anode is positive and the cathode negative; the metal dissolves off the anode and is deposited on the cathode. Getting this the right way round decides Questions 23, 24 and 36.

Key insight. Questions 29 and 30 are the same question in disguise: given electronic configurations or given which atom lost the electron, you are being asked to spot a metal reacting with a non-metal, and therefore an ionic compound — and once you have said “ionic”, every property follows without further thought.

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

Question 19

Generally, non-metals are not lustrous. Which of the following non-metal is lustrous?

Solution. Lustre is a metallic property, so the question is asking for the exception among non-metals — and iodine is it. Solid iodine forms shiny, grey-black crystals with a distinctly metallic sheen.

Oxygen and nitrogen can be eliminated before any chemistry is done: both are colourless gases at room temperature, and lustre is a property of a solid surface. Sulphur is a solid, but it is a dull yellow powder.

Iodine and graphite are the two lustrous non-metals the syllabus asks you to remember, and they sit alongside the other standard exceptions worth keeping together: graphite conducts electricity (Question 32), diamond has an extremely high melting point and is the hardest substance known, and bromine is a liquid (Question 34). Each is a non-metal behaving as a metal would.

Question 20

Which one of the following four metals would be displaced from the solution of its salts by other three metals?

Solution. A metal displaces another from its salt solution only if it is more reactive. So a metal that is displaced by all three of the others must be the least reactive of the four.

Order the options by the reactivity series:

$$\mathrm{Mg} > \mathrm{Zn} > \mathrm{Cu} > \mathrm{Ag}$$

Silver is at the bottom, so magnesium, zinc and copper can all displace it from, say, silver nitrate solution:

$$\mathrm{Cu}(s) + 2\mathrm{AgNO_3}(aq) \rightarrow \mathrm{Cu(NO_3)_2}(aq) + 2\mathrm{Ag}(s)$$

Magnesium is at the top and would be displaced by none of them. This is the standard shape of the question — sort the options by reactivity first, then read whether the question wants the top or the bottom of the list, and answer in a second.

Question 21

2 mL each of concentrated HCl, $\mathrm{HNO_3}$ and a mixture of concentrated HCl and concentrated $\mathrm{HNO_3}$ in the ratio of 3 : 1 were taken in test tubes labelled as A, B and C. A small piece of metal was put in each test tube. No change occurred in test tubes A and B but the metal got dissolved in test tube C respectively. The metal could be

Solution. Test tube C contains aqua regia, from Question 8. The metal is therefore one that resists both concentrated acids individually but dissolves in the mixture.

Aluminium is out at once: it reacts vigorously with hydrochloric acid, so test tube A would have fizzed. Copper is out too: it is unreactive towards hydrochloric acid, but nitric acid attacks it readily as an oxidising agent, so test tube B would have turned blue.

Gold fits perfectly — it is below hydrogen in the reactivity series and unaffected by either acid alone, and aqua regia is the classic reagent for dissolving it. That is the key’s answer.

A note on option (d). Platinum behaves the same way. It too resists both concentrated acids and dissolves in aqua regia — the mixture is named “royal water” precisely because it dissolves the two royal metals, gold and platinum. Read strictly, this question has two correct options. Gold is the answer the exemplar wants, because gold is the example the textbook gives; but if this appeared in a board paper with platinum among the choices, it would be a defective question rather than a hard one.

Question 22

An alloy is

Solution. An alloy is made by melting metals together and letting them solidify, so the components are mixed but not chemically combined. That rules out (a) and (b): it is not an element, since more than one kind of atom is present, and it is not a compound, because the components are not in a fixed proportion and no new chemical bonds join them into a new substance. Brass can be made with a range of copper-to-zinc ratios and is still brass.

It is a homogeneous mixture because the metals dissolve in one another while molten and stay uniformly distributed on solidifying — a sample from any part of a brass rod has the same composition as any other. A heterogeneous mixture would have visibly distinguishable parts.

The reason alloys matter is that mixing changes physical properties without any chemistry: pure iron is soft, steel is hard; pure gold is too soft for jewellery, so it is alloyed with copper or silver.

Question 23

An electrolytic cell consists of

(i) positively charged cathode (ii) negatively charged anode (iii) positively charged anode (iv) negatively charged cathode

Solution. In an electrolytic cell the electrodes take their charge from the battery they are connected to. The anode is joined to the positive terminal and the cathode to the negative terminal, so:

• anode is positive — statement (iii)
• cathode is negative — statement (iv)

which is option (b). Statements (i) and (ii) are those two reversed, so options (a), (c) and (d) each contain at least one of them.

The reason the charges are that way round is worth holding, because it explains the whole of Question 36. The positive anode attracts the negative ions, and the negative cathode attracts the positive metal ions — which is why the metal is deposited at the cathode and why refining puts the pure metal there.

One caution: in a galvanic cell, which makes electricity rather than consuming it, the polarity is the other way round. The chapter deals only with electrolytic cells, but the rule is not universal.

Question 24

During electrolytic refining of zinc, it gets

Solution. Electrolytic refining works by taking the metal apart and putting it back together somewhere cleaner.

The impure metal is the anode. Being positive, it loses electrons and dissolves into the electrolyte as ions:

$$\mathrm{Zn}(s) \rightarrow \mathrm{Zn^{2+}}(aq) + 2e^-$$

Those $\mathrm{Zn^{2+}}$ ions are attracted to the negative cathode, where they gain electrons and are deposited as pure zinc:

$$\mathrm{Zn^{2+}}(aq) + 2e^- \rightarrow \mathrm{Zn}(s)$$

So the pure metal builds up on the cathode, which is option (a). The impurities, which are either less reactive or insoluble, do not dissolve and settle beneath the anode as anode mud — and the valuable ones, such as the silver and gold in impure copper, are recovered from it.

Option (d) misses that the ions are in the solution only in transit; the process is continuous, and the zinc ends up as solid metal.

Question 25

An element A is soft and can be cut with a knife. This is very reactive to air and cannot be kept open in air. It reacts vigorously with water. Identify the element from the following

Solution. Three clues, and taken together they point at one element.

Soft enough to cut with a knife is unusual for a metal and is the textbook description of the alkali metals — sodium and potassium. Magnesium and calcium are noticeably harder.

Reacts vigorously with water confirms a metal near the top of the reactivity series. Sodium does so violently enough that the hydrogen catches fire; calcium reacts, but gently, as Question 6 established; magnesium needs hot water.

Cannot be kept open in air is the reason sodium is stored under kerosene — it would otherwise react with the oxygen and moisture of the air at once.

Phosphorus is the distractor worth a moment. White phosphorus is indeed soft and so reactive it must be stored under water — but it is a non-metal, and it does not react with water at all; it is stored under water precisely because it does not. The word element rather than metal in the question is what lets phosphorus onto the list.

Question 26

Alloys are homogeneous mixtures of a metal with a metal or non-metal. Which among the following alloys contain non-metal as one of its constituents?

Solution. Write out what each alloy is made of and the answer is immediate:

• Brass — copper and zinc. Both metals.
• Bronze — copper and tin. Both metals.
• Amalgam — a metal dissolved in mercury. Both metals, by definition.
• Steel — iron and carbon. Carbon is a non-metal.

So steel is the answer. The carbon content is small — well under two per cent — but it transforms the metal: pure iron is soft and easily bent, while steel is hard and strong, and the hardness can be tuned by how much carbon is added.

Note that stainless steel, from Question 15, is steel with nickel and chromium added on top of that carbon, so it also contains a non-metal. The question offers plain steel, which is the cleanest example.

Question 27

Which among the following statements is incorrect for magnesium metal?

Solution. Statement (b) is wrong twice over, which is what makes it the answer.

First, magnesium does not react appreciably with cold water. It belongs to the hot-water band of Question 4 — this is the single fact the question is testing.

Second, even where magnesium does react with water, the product is the hydroxide, not the oxide:

$$\mathrm{Mg}(s) + 2\mathrm{H_2O}(l) \xrightarrow{\text{hot}} \mathrm{Mg(OH)_2}(aq) + \mathrm{H_2}(g)$$

Metals reacting with water give hydroxides; the oxide is what you get by burning the metal in oxygen, which is statement (a):

$$2\mathrm{Mg}(s) + \mathrm{O_2}(g) \rightarrow 2\mathrm{MgO}(s)$$

and that reaction really does give the dazzling white light that makes magnesium ribbon a classroom favourite and once made it the basis of flash photography.

Statements (c) and (d) are both correct as written.

Question 28

Which among the following alloys contain mercury as one of its constituents?

Solution. The definition settles it: an amalgam is any alloy in which one of the metals is mercury. The word carries the information, so zinc amalgam is zinc dissolved in mercury.

The others contain no mercury. Stainless steel is iron with nickel and chromium; alnico, as its name compresses, is aluminium, nickel and cobalt with iron, and is used for permanent magnets; solder is lead and tin.

Amalgams are possible because mercury is liquid at room temperature — Question 17 — so other metals can simply dissolve in it as a solid dissolves in a solvent. That is how dental amalgam is made and how gold was once extracted from crushed ore.

Question 29

Reaction between X and Y, forms compound Z. X loses electron and Y gains electron. Which of the following properties is not shown by Z?

Solution. Electrons have been transferred — X loses, Y gains — so Z is an ionic compound. Once that word is said, every option can be checked against the standard family of ionic properties from Question 10.

$\mathrm{X^+}$ and $\mathrm{Y^-}$ attract each other electrostatically, and in the solid every ion is held by all its neighbours in a rigid three-dimensional lattice. Breaking that lattice takes a great deal of energy, so ionic compounds have high melting and boiling points and are solid at room temperature. Statements (a) and (d) are true.

Melting frees the ions to move, so the molten compound conducts. Statement (c) is true.

Statement (b) contradicts (a) and is the one property Z does not have.

Notice that (a) and (b) are a contradictory pair, so one of them must be the answer before any chemistry is done; the chemistry only decides which.

Question 30

The electronic configurations of three elements X, Y and Z are X — 2, 8; Y — 2, 8, 7 and Z — 2, 8, 2. Which of the following is correct?

Solution. Everything is decided by the number of electrons in the outermost shell, because that is what an atom uses to bond.

X — 2, 8. Eight electrons in the outer shell is a complete octet. X has no tendency to gain or lose electrons at all: it is a noble gas (neon). It is neither a metal nor a non-metal in the reactive sense, so statement (a) is false.

Y — 2, 8, 7. Seven in the outer shell, one short of eight. Y will gain an electron, which is what non-metals do. It is a non-metal (chlorine), so statement (b) is false.

Z — 2, 8, 2. Two in the outer shell, easily lost. Z will lose electrons, which is what metals do. It is a metal (magnesium), so statement (c) is false.

Only statement (d) gets both right, and it also describes exactly the pair that would form an ionic compound — a metal losing to a non-metal, as in Question 29. Z and Y would give $\mathrm{ZY_2}$, which is magnesium chloride.

The rule to carry: one to three outer electrons means a metal; four to seven means a non-metal; eight means a noble gas.

Question 31

Although metals form basic oxides, which of the following metals form an amphoteric oxide?

Solution. An amphoteric oxide reacts with acids and with bases, giving a salt and water in both cases. Aluminium oxide is the standard example the syllabus asks for:

$$\mathrm{Al_2O_3} + 6\mathrm{HCl} \rightarrow 2\mathrm{AlCl_3} + 3\mathrm{H_2O}$$

$$\mathrm{Al_2O_3} + 2\mathrm{NaOH} \rightarrow 2\mathrm{NaAlO_2} + \mathrm{H_2O}$$

Behaving as a base in the first and as an acid in the second is precisely what amphoteric means.

Sodium oxide and calcium oxide are strongly basic and do nothing with alkalis. Copper oxide is basic too.

The other amphoteric oxide worth remembering is zinc oxide, which is why zinc gives hydrogen with sodium hydroxide as well as with acids — the fact behind Question 43(d) and Question 48 of the previous unit. Aluminium and zinc are the two amphoteric metals of the Class 10 syllabus.

Question 32

Generally, non-metals are not conductors of electricity. Which of the following is a good conductor of electricity?

Solution. Graphite is the standard exception, and the reason lies in its structure. Each carbon atom in graphite is bonded to only three neighbours, in flat hexagonal sheets. That leaves one electron per atom not used in bonding, and those spare electrons are free to move along the sheet — which is what conduction is.

Diamond is the same element arranged differently, and the contrast is the point. Each carbon there is bonded to four neighbours in a rigid three-dimensional network, so every electron is committed to a bond and none is free. Diamond is an excellent insulator.

Sulphur is a non-conductor. Fullerene is a third form of carbon, a closed cage of sixty atoms; it is not a good conductor in the ordinary sense, and it is on the list to test whether “it is carbon” alone is being used as the reason.

Graphite’s conductivity is what makes it useful as an electrode, including in the electrolytic cell of Question 36.

Question 33

Electrical wires have a coating of an insulting material. The material, generally used is

Solution. The coating has to be an electrical insulator, and it also has to be flexible, durable and cheap enough to use by the kilometre.

Graphite is eliminated on the chemistry: Question 32 has just established that it conducts electricity, so coating a wire in it would be worse than useless.

Sulphur is an insulator, so it fails on the practical grounds rather than the chemical ones. It is a brittle yellow solid that would crack off the moment the wire was bent, it melts at a low temperature, and it burns.

PVC — polyvinyl chloride — is the material actually used. It is an excellent insulator, flexible, tough, resistant to water and chemicals, and cheap to extrude straight onto the wire.

Option (d) cannot be right once graphite is on the list.

Question 34

Which of the following non-metals is a liquid?

Solution. Bromine is the only non-metal that is liquid at room temperature — a dense, dark red-brown liquid that gives off a choking orange vapour.

Carbon, phosphorus and sulphur are all solids. Carbon in particular has an extraordinarily high melting point in both its common forms.

This question pairs with Question 17. Among all the elements, only two are liquid at room temperature: mercury, which is a metal, and bromine, which is a non-metal. Question 52 in the short-answer band asks for both at once, so they are worth learning as a pair rather than separately.

Question 35

Which of the following can undergo a chemical reaction?

Solution. Each option is a metal and a salt of another metal, so each is a possible displacement reaction — and each happens only if the free metal is more reactive than the metal in the salt.

Reading the four against the series $\mathrm{Mg} > \mathrm{Zn} > \mathrm{Fe} > \mathrm{Pb} > \mathrm{Cu}$:

• (a) Iron against magnesium — iron is less reactive. No reaction.
• (b) Iron against zinc — iron is less reactive. No reaction.
• (c) Lead against magnesium — lead is far less reactive. No reaction.
• (d) Iron against copper — iron is more reactive. Reaction.

$$\mathrm{Fe}(s) + \mathrm{CuSO_4}(aq) \rightarrow \mathrm{FeSO_4}(aq) + \mathrm{Cu}(s)$$

The blue solution fades to the pale green of iron sulphate and a red-brown deposit of copper appears. This is the reaction that ruins the iron pot in Question 58 of the short-answer band — the iron dissolves away and leaves holes.

Question 36

Which one of the following figures correctly describes the process of electrolytic refining?

In every panel the beaker holds acidified $\mathrm{CuSO_4}$ solution and the dark mound on the floor of the beaker is the impurities.

Solution. All four pictures look almost identical, so the way through is to write down what a correct diagram must show and then check the four against the list. There are exactly four things that can differ, and a correct figure needs all four right.

1. The anode must be joined to the positive terminal and the cathode to the negative — Question 23.
2. The impurities must lie under the anode, because that is the electrode that dissolves away and leaves them behind.
3. The $\mathbf{Cu^{2+}}$ ions must travel from anode to cathode, since they leave the anode and are deposited on the cathode — Question 24.
4. The electrolyte must be a solution of a salt of the metal, which all four have.

Now take the panels in turn.

(a) The battery’s $+$ is on the left, and the left electrode is labelled cathode. That is backwards at the first test, so (a) fails, even though its impurities and arrow are correctly placed.

(b) The polarity and the labels agree — anode on the left with $+$, cathode on the right with $-$ — and the ions travel left to right, from anode to cathode, correctly. But the impurities are drawn under the right-hand electrode, which is the cathode. The cathode gains pure metal; nothing settles beneath it. (b) fails on the second test.

(c) The $-$ is on the left and the left electrode is the cathode ✓. The $+$ is on the right and the right electrode is the anode ✓. The impurities lie under that right-hand anode ✓. And the $\mathrm{Cu^{2+}}$ arrow points left, from the anode towards the cathode ✓. All four right.

(d) Identical to (c) except that the ion arrow points the wrong way — from the cathode towards the anode. Since the copper is supposed to be transferred to the cathode, that reverses the whole process.

So (c) is the correct figure.

Common mistakes

• Reversing the electrodes, in Questions 23, 24 and 36. The anode is positive and dissolves; the cathode is negative and gains the pure metal; the impurities settle under the anode. Three questions in this half turn on the same three facts.
• Answering Question 36 on a general impression. The four figures differ only in polarity, labels, arrow direction and impurity position. A figure question of this shape has to be answered by checking a written list against each panel, not by recognising the “right-looking” one.
• Saying magnesium reacts with cold water, in Question 27. It does not; it needs hot water. And metals with water give hydroxides — the oxide is what burning in oxygen gives.
• Assuming any form of carbon conducts, in Questions 32 and 33. Graphite conducts because one electron per atom is left free; diamond does not, because all four are committed to bonds. Same element, opposite behaviour.
• Calling an alloy a compound, in Question 22. There is no fixed proportion and no chemical bond between the components — it is a homogeneous mixture, which is why the properties can be tuned by changing the ratio.
• Forgetting aluminium and zinc are the amphoteric pair, in Question 31. Their oxides react with acids and alkalis. It is the fact behind zinc giving hydrogen with sodium hydroxide.
• Reading outer-shell counts loosely in Question 30. One to three outer electrons is a metal, four to seven a non-metal, eight a noble gas. A configuration ending in 8 is not a “non-metal with lots of electrons”.
• Missing that Question 21 has two defensible answers. Platinum resists both concentrated acids and dissolves in aqua regia exactly as gold does. Gold is the answer expected, but the question as printed does not exclude platinum.

Practise next

• Exemplar Multiple Choice Questions Q1–18 — the first half of this band, on physical properties and the reactions with water and acids that Questions 20, 27 and 35 above assume.
• Exemplar Short and Long Answer Questions — where the refining cell of Question 36 has to be drawn and labelled from scratch, in Question 62.
• NCERT textbook Chapter 3 — Metals and Non-metals for the end-of-chapter exercise on ionic bonding, alloys and extraction.
• Textbook activities for the displacement experiments behind Questions 20 and 35, where the colour changes these questions describe are actually observed.