Chapter 3 · Metals and Non-metals

NCERT Class 10 Science Chapter 3 Exercises

All 16 questions of NCERT Class 10 Science Chapter 3 Exercises (Metals and Non-metals), in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Which ​of the following ​pairs will give ​displacement ​reactions?

   1. (a)$\text{NaCl}$ solution and copper metal
   2. (b)${\text{MgCl}}_2$ solution and aluminium metal
   3. (c)${\text{FeSO}}_4$ solution and silver metal
   4. (d)${\text{AgNO}}_3$ solution and copper metal.
   Show answer

   (d) ${\text{AgNO}}_3$ solution and copper metal.

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2. Q2
   Which of the ​following methods is ​suitable ​for ​preventing an iron frying ​pan ​from rusting?

   1. (a)Applying grease
   2. (b)Applying paint
   3. (c)Applying a coating of zinc
   4. (d)All of the above.
   Show answer

   (c) Applying a coating of zinc

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3. Q3
   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

   1. (a)calcium
   2. (b)carbon
   3. (c)silicon
   4. (d)iron.
   Show answer

   (a) calcium

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4. Q4
   Food cans ​are ​coated ​with tin ​and not ​with zinc ​because

   1. (a)zinc is costlier than tin.
   2. (b)zinc has a higher melting point than tin.
   3. (c)zinc is more reactive than tin.
   4. (d)zinc is less reactive than tin.
   Show answer

   (c) zinc is more reactive than tin.

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5. Q5
   You ​are given a hammer, a ​battery, a bulb, wires and a switch. ​

   1. (a)How could you use ​them to distinguish between samples of ​metals and non-metals?
   2. (b)Assess the ​usefulness of these ​tests in distinguishing between metals and non-metals.
   Show answer

   Metals flatten under a hammer and light the bulb, non-metals break and do not; exceptions make the tests only a quick first sort.

   (a) Hammer each sample: a metal is malleable and flattens into a sheet, while a non-metal is brittle and breaks into pieces. Then join the battery, bulb and switch with the wires into a circuit that has a gap in it, clip the sample across that gap and close the switch: if the bulb glows the sample conducts electricity and is a metal, and if it stays dark the sample is a non-metal. (b) Taken together the two tests sort the common elements correctly, but neither is decisive. Graphite is a non-metal that conducts and would light the bulb, and some metals — zinc, for instance — are brittle at ordinary temperature and shatter under the hammer. Both tests also need a solid sample, so they say nothing at all about non-metals that are gases or liquids. They are therefore a quick first sort rather than a proof; the reliable distinction is the chemical one, that metals form basic oxides while non-metals form acidic oxides.

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6. Q6
   What are ​amphoteric oxides? ​Give two ​examples of amphoteric ​oxides.
   Show answer

   Amphoteric oxides are metal oxides that react with both acids and bases, e.g. aluminium oxide and zinc oxide.

   Amphoteric oxides are metal oxides that react with both acids and bases to produce a salt and water — they show basic behaviour towards an acid and acidic behaviour towards a base. Two examples are aluminium oxide, ${\text{Al}}_2{\text{O}}_3$, and zinc oxide, $\text{ZnO}$: ${\text{Al}}_2{\text{O}}_3+6\text{HCl}⟶2{\text{AlCl}}_3+3{\text{H}}_2\text{O}$ and ${\text{Al}}_2{\text{O}}_3+2\text{NaOH}⟶2{\text{NaAlO}}_2+{\text{H}}_2\text{O}$; $\text{ZnO}+2\text{HCl}⟶{\text{ZnCl}}_2+{\text{H}}_2\text{O}$ and $\text{ZnO}+2\text{NaOH}⟶{\text{Na}}_2{\text{ZnO}}_2+{\text{H}}_2\text{O}$.

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7. Q7
   Name two metals ​which will ​displace ​hydrogen from dilute ​acids, and ​two metals ​which will ​not.
   Show answer

   Zinc and magnesium displace hydrogen from dilute acids; copper and silver do not, as they lie below hydrogen in the reactivity series.

   Zinc and magnesium displace hydrogen from dilute acids, because both lie above hydrogen in the reactivity series: $\text{Zn}+2\text{HCl}⟶{\text{ZnCl}}_2+{\text{H}}_2$ and $\text{Mg}+2\text{HCl}⟶{\text{MgCl}}_2+{\text{H}}_2$. Copper and silver will not, because both lie below hydrogen, so a strip of either left in dilute hydrochloric acid gives no reaction at all. (Iron, aluminium and calcium would do equally well for the first pair, and gold or platinum for the second.)

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8. Q8
   In the electrolytic ​refining ​of ​a ​metal M, ​what would you take as the anode, the cathode and the electrolyte?
   Show answer

   The anode is impure M, the cathode is pure M and the electrolyte is a solution of a salt of M.

   Anode: the impure metal M, taken as a thick block. Cathode: a thin strip of pure metal M. Electrolyte: an aqueous solution of a salt of M — for example its sulphate, usually acidified with the corresponding acid to make the solution conduct better. On passing the current, M dissolves from the anode as ${\text{M}}^{n+}$ ions and an equal amount of pure M is deposited on the cathode; the soluble impurities remain in solution and the insoluble ones settle below the anode as anode mud.

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9. Q9
   Pratyush took sulphur powder on a spatula and heated it. He collected ​the gas evolved by ​inverting a test tube over it, ​as ​shown in figure below. (a) What will be the action ​of gas on ​

   1. (i)dry litmus ​paper? ​
   2. (ii)moist litmus paper? (b) Write a balanced chemical equation for the reaction taking place.

   Show answer

   Dry litmus paper shows no change; moist blue litmus turns red as ${\text{SO}}_2$ forms sulphurous acid; $\text{S}+{\text{O}}_2\to {\text{SO}}_2$.

   (a)(i) Dry litmus paper: no change. Sulphur dioxide is an acidic oxide but not an acid, and with no water present it releases no hydrogen ions, so neither blue nor red dry litmus alters. (a)(ii) Moist litmus paper: moist **blue** litmus turns **red** (moist red litmus stays red). The gas dissolves in the water on the paper to give sulphurous acid, ${\text{H}}_2{\text{SO}}_3$, whose hydrogen ions redden the dye. (b) $\text{S}(s)+{\text{O}}_2(g)⟶{\text{SO}}_2(g)$, and in the moisture on the paper ${\text{SO}}_2(g)+{\text{H}}_2\text{O}(l)⟶{\text{H}}_2{\text{SO}}_3(aq)$.

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10. Q10
    State ​two ​ways ​to prevent ​the ​rusting of iron.
    Show answer

    Coat the iron with paint, grease or oil, or galvanise it with a layer of zinc.

    Two ways: (1) Coat the surface with paint, grease or oil. The film is a barrier that stops air and moisture from touching the iron, and rusting cannot occur without both of them. (2) Galvanise the iron, that is, coat it with a thin layer of zinc. Zinc keeps air and water off the iron, and because zinc is more reactive than iron it corrodes in the iron's place, so the protection survives even where the coat is scratched. Electroplating with chromium or nickel, and alloying the iron into stainless steel, are equally acceptable alternatives.

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11. Q11
    What ​type of oxides are ​formed ​when non-metals ​combine ​with ​oxygen?
    Show answer

    Non-metals form acidic oxides, which dissolve in water to give acids.

    Non-metals combine with oxygen to form **acidic** oxides (acid anhydrides). Each dissolves in water to give an acid and turns moist blue litmus red — for example ${\text{SO}}_2$ gives sulphurous acid, ${\text{SO}}_2+{\text{H}}_2\text{O}⟶{\text{H}}_2{\text{SO}}_3$; ${\text{CO}}_2$ gives carbonic acid, ${\text{CO}}_2+{\text{H}}_2\text{O}⟶{\text{H}}_2{\text{CO}}_3$; and ${\text{P}}_2{\text{O}}_5$ gives phosphoric acid. They also neutralise bases, as in ${\text{CO}}_2+2\text{NaOH}⟶{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}$. A few non-metal oxides are an exception and are **neutral** — carbon monoxide $\text{CO}$, nitrous oxide ${\text{N}}_2\text{O}$, nitric oxide $\text{NO}$ and water ${\text{H}}_2\text{O}$.

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12. Q12
    Give reasons ​

    1. (a)Platinum, gold and silver are ​used to make jewellery.
    2. (b)Sodium, potassium and lithium are stored under oil.
    3. (c)Aluminium is a highly ​reactive metal, ​yet it ​is ​used to make utensils for cooking.
    4. (d)Carbonate and sulphide ​ores are usually converted into oxides during the process of extraction.
    Show answer

    Unreactive metals do not corrode; very reactive ones are kept away from air and water; aluminium is sealed by its oxide; oxides reduce easily.

    (a) Platinum, gold and silver lie at the bottom of the reactivity series, so they do not react with air, water or most acids and never corrode — their lustre lasts. They are also highly malleable and ductile, so they can be worked into ornaments. (b) Sodium, potassium and lithium are so reactive that they react at once with the oxygen and moisture of the air, and their reaction with water is violent enough to set the hydrogen released on fire: $2\text{Na}+2{\text{H}}_2\text{O}⟶2\text{NaOH}+{\text{H}}_2+\text{heat}$. Kerosene oil keeps air and moisture off them and does not itself react. (c) Aluminium does react with oxygen, $4\text{Al}+3{\text{O}}_2⟶2{\text{Al}}_2{\text{O}}_3$, but the oxide forms a thin, hard layer that sticks to the surface and seals it, so no further attack is possible. Together with its good thermal conductivity, high melting point and low density, that makes it safe for cooking utensils. (d) Because a metal is much more easily obtained by reducing its oxide than its carbonate or sulphide. The carbonate ore is calcined, ${\text{ZnCO}}_3\stackrel{\Delta }{\to }\text{ZnO}+{\text{CO}}_2$, and the sulphide ore is roasted, $2\text{ZnS}+3{\text{O}}_2\stackrel{\Delta }{\to }2\text{ZnO}+2{\text{SO}}_2$; the oxide is then reduced with carbon.

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13. Q13
    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.
    Show answer

    Citric acid in lemon and tartaric acid in tamarind neutralise the basic copper carbonate coating, forming a soluble salt that washes away.

    A copper vessel standing in moist air becomes coated with a dull green layer of basic copper carbonate, $2\text{Cu}+{\text{H}}_2\text{O}+{\text{CO}}_2+{\text{O}}_2⟶{\text{CuCO}}_3\cdot {\text{Cu(OH)}}_2$, and this coating is what makes the vessel look tarnished. Lemon juice contains citric acid and tamarind juice contains tartaric acid, which is why both taste sour. Being acids, they react with the **basic** copper carbonate and neutralise it, giving a soluble copper salt together with water and carbon dioxide — the pattern is that of any carbonate with an acid, ${\text{CuCO}}_3\cdot {\text{Cu(OH)}}_2+4\text{HCl}⟶2{\text{CuCl}}_2+3{\text{H}}_2\text{O}+{\text{CO}}_2$. The soluble salt washes away, the bare copper beneath is exposed, and the vessel shines again. The mild acid does not attack the copper metal itself, since copper lies below hydrogen in the reactivity series.

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14. Q14
    Differentiate between metal ​and non-metal ​on ​the basis of ​their chemical ​properties.
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    Metals lose electrons and form basic oxides and ionic compounds; non-metals gain electrons and form acidic oxides and covalent compounds.

    Metals and non-metals differ on every chemical property. (1) With oxygen: metals form **basic** oxides — $2\text{Mg}+{\text{O}}_2⟶2\text{MgO}$, and ${\text{Na}}_2\text{O}+{\text{H}}_2\text{O}⟶2\text{NaOH}$ turns red litmus blue (a few, such as ${\text{Al}}_2{\text{O}}_3$ and $\text{ZnO}$, are amphoteric); non-metals form **acidic** oxides — $\text{S}+{\text{O}}_2⟶{\text{SO}}_2$, and ${\text{SO}}_2+{\text{H}}_2\text{O}⟶{\text{H}}_2{\text{SO}}_3$ turns blue litmus red (a few, such as $\text{CO}$ and ${\text{N}}_2\text{O}$, are neutral). (2) With water: reactive metals react, giving hydrogen — $2\text{Na}+2{\text{H}}_2\text{O}⟶2\text{NaOH}+{\text{H}}_2$ and $3\text{Fe}+4{\text{H}}_2\text{O}⟶{\text{Fe}}_3{\text{O}}_4+4{\text{H}}_2$ with steam; non-metals do not react with water. (3) With dilute acids: metals above hydrogen in the reactivity series displace hydrogen — $\text{Zn}+2\text{HCl}⟶{\text{ZnCl}}_2+{\text{H}}_2$; non-metals do not react and never displace hydrogen. (4) With salt solutions: a more reactive metal displaces a less reactive metal — $\text{Fe}+{\text{CuSO}}_4⟶{\text{FeSO}}_4+\text{Cu}$; a more reactive non-metal displaces a less reactive non-metal — ${\text{Cl}}_2+2\text{NaBr}⟶2\text{NaCl}+{\text{Br}}_2$ — but neither family displaces the other. (5) With chlorine: metals give **ionic** chlorides such as $\text{NaCl}$ and ${\text{MgCl}}_2$, which are high-melting solids that conduct when molten; non-metals give **covalent** chlorides such as $\text{HCl}$ and ${\text{PCl}}_3$, which are volatile and do not conduct. (6) With hydrogen: only the most reactive metals combine, forming **ionic** hydrides such as $\text{NaH}$ and ${\text{CaH}}_2$; non-metals combine readily, forming **covalent** hydrides such as ${\text{NH}}_3$, $\text{HCl}$ and ${\text{H}}_2\text{S}$. Underlying all six: a metal loses electrons and is electropositive, a reducing agent forming positive ions; a non-metal gains electrons and is electronegative, an oxidising agent forming negative ions.

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15. Q15
    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. The ​lady was upset but after a ​futile argument the man ​beat a hasty retreat. Can you play the detective to find out the nature ​of the solution he had used?
    Show answer

    Aqua regia, a $3:1$ mixture of concentrated HCl and ${\text{HNO}}_3$, dissolved a layer of gold, so the bangles shone but lost weight.

    The solution was **aqua regia** — a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio $3:1$ (three parts $\text{HCl}$ to one part ${\text{HNO}}_3$). It is a fuming, highly corrosive liquid and one of the very few reagents able to dissolve gold, which no single acid attacks because gold lies at the bottom of the reactivity series. When the bangles were dipped in it, the outer layer of gold dissolved away: the freshly exposed surface underneath looked bright and new, which is why they sparkled, while the gold that had gone into the solution was lost from the ornaments, which is why their weight fell drastically. The man kept that dissolved gold — the 'polishing' was a theft.

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16. Q16
    Give ​reasons ​why ​copper is used to make ​hot ​water tanks and not steel ​(an alloy of ​iron).
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    Copper does not react with water or steam, but iron in steel reacts with steam and rusts, so a steel tank would corrode.

    Because copper does not react with water while iron does. Copper lies below hydrogen in the reactivity series and does not react with cold water, hot water or even steam, so a copper tank is left chemically untouched by the hot water it holds and does not contaminate it. Steel is an alloy of iron, and iron reacts with steam, $3\text{Fe}+4{\text{H}}_2\text{O}⟶{\text{Fe}}_3{\text{O}}_4+4{\text{H}}_2$, as well as rusting in the presence of air and moisture. A steel tank would therefore corrode from the inside: the wall would thin, weaken and eventually leak, the iron oxide would discolour and spoil the water, and the hydrogen set free is flammable. Copper also conducts heat very well, so the water heats quickly, and its melting point of about $1085{}^{\circ }$C is far above any temperature a hot water tank reaches.

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