Class 10 Science · Chapter 1

Chemical Reactions and Equations

201 Class 10 Science questions on Chemical Reactions and Equations from the NCERT textbook, NCERT Exemplar and CBSE board papers 2024–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 2 of 11

1. Q21 NCERT · Exercises
   Why ​do ​we apply paint on iron ​articles?
   Show answer

   Paint forms a continuous film that cuts off air and moisture, so the iron cannot rust.

   Paint is applied to iron articles to prevent rusting, which is the corrosion of iron. Iron rusts only when oxygen of the air and moisture reach its surface together, as the equation $4\text{Fe}(s)+3{\text{O}}_2(g)+2x{\text{H}}_2\text{O}(l)\to 2{\text{Fe}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}(s)$ shows. A coat of paint forms a continuous impervious film over the metal and cuts off both air and moisture at once, so the reaction cannot begin. This matters because rust is flaky and porous and does not protect the iron beneath it: once rusting starts it eats steadily deeper and weakens the article, so a large amount of iron would otherwise be lost. The protection is purely a physical barrier, which is why rusting begins at any point where the paint is scratched or chipped.

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2. Q22 NCERT · Exercises
   Oil ​and ​fat ​containing ​food ​items are flushed ​with nitrogen. Why?
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   Nitrogen is unreactive and drives out oxygen, so the fats and oils cannot be oxidised and turn rancid.

   Oils and fats are slowly oxidised by the oxygen of the air, and the oxidation products give the food an unpleasant smell and taste — the food is said to have become rancid. Nitrogen is an unreactive gas that does not attack fats and oils, so flushing the packet with it drives out the air and leaves almost no oxygen in contact with the food. With the reactant removed, the oxidation cannot take place, the food is kept from turning rancid, and its shelf life is extended. Nitrogen is not itself a preservative — it protects only by occupying the space that oxygen would otherwise fill.

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3. Q23 NCERT · Exercises
   Explain the ​following ​terms ​with one example ​each.

   1. (a)​Corrosion
   2. (b)​Rancidity
   Show answer

   Corrosion, e.g. rusting of iron, slowly eats away a metal; rancidity, e.g. in stale butter, is oxidation of fats and oils.

   (a) Corrosion is the slow eating away of a metal surface by the action of the air, moisture or chemicals around it, in which the metal at the surface is converted into a compound. Example: the rusting of iron, in which an iron article exposed to damp air becomes coated with flaky reddish-brown rust, $4\text{Fe}(s)+3{\text{O}}_2(g)+2x{\text{H}}_2\text{O}(l)\to 2{\text{Fe}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}(s)$. (b) Rancidity is the spoiling of fats and oils in a food by oxidation on standing in air, which changes their smell and taste and makes them unpleasant. Example: butter or fried snacks left for a long time in an open container develop a bad smell and taste. Both are slow oxidation by atmospheric oxygen, and both are prevented by keeping the air away — by painting or oiling the metal, and by airtight packing, antioxidants or flushing with nitrogen for the food.

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4. Q24 NCERT · Intext Questions (Page 10)
   A solution of a ​substance ‘X’ is used for whitewashing.

   1. (i)​Name ​the ​substance ‘X’ ​and write its ​formula.
   2. (ii)Write the reaction of ​the substance ‘X’ ​named in (i) above with water.
   Show answer

   $X$ is calcium oxide, $\text{CaO}$, which reacts with water to give calcium hydroxide and a large amount of heat.

   (i) $X$ is calcium oxide (quick lime), formula $\text{CaO}$; its solution in water (slaked lime solution) is used for whitewashing. (ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime) with the release of a large amount of heat: $\text{CaO}(s)+{\text{H}}_2\text{O}(l)\to {\text{Ca(OH)}}_2(aq)+\text{heat}$. This is an exothermic combination reaction.

   Step-by-step solution →
5. Q25 NCERT · Intext Questions (Page 10)
   Why ​is the ​amount ​of gas collected in one ​of the ​test tubes in ​Activity 1.7 double of the ​amount collected in the other? Name this gas.
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   Electrolysis of water gives hydrogen and oxygen in a 2 : 1 volume ratio, so the gas in double amount is hydrogen.

   Activity 1.7 is the electrolysis of water: $2{\text{H}}_2\text{O}(l)\stackrel{\text{electric current}}{\to }2{\text{H}}_2(g)+{\text{O}}_2(g)$. Water contains hydrogen and oxygen atoms in the ratio $2:1$, so on decomposition two molecules — and hence two volumes — of hydrogen are produced for every one of oxygen. The gas collected in double the amount is therefore hydrogen (${\text{H}}_2$), which forms at the cathode; oxygen, collected at the anode, has half its volume.

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6. Q26 NCERT · Intext Questions (Page 13)
   Why does ​the ​colour ​of copper sulphate solution ​change when an iron ​nail ​is dipped in ​it?
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   Iron displaces copper from the blue solution, forming light-green iron(II) sulphate, so the colour changes from blue to green.

   Iron is more reactive than copper, so when an iron nail is dipped in copper sulphate solution it displaces copper from the solution: $\text{Fe}(s)+{\text{CuSO}}_4(aq)\to {\text{FeSO}}_4(aq)+\text{Cu}(s)$. The blue colour of the solution is due to copper sulphate; as it is used up and light-green iron(II) sulphate (ferrous sulphate) forms in its place, the colour changes from blue to green, and the nail becomes brownish because of the copper deposited on it. This is a displacement reaction.

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7. Q27 NCERT · Intext Questions (Page 13)
   Give an example of ​a double ​displacement ​reaction ​other than the ​one ​given ​in Activity 1.10.
   Show answer

   Lead nitrate with potassium iodide gives yellow lead iodide as the ions exchange partners: $\text{Pb}({\text{NO}}_3{)}_2+2\text{KI}\to {\text{PbI}}_2+2{\text{KNO}}_3$.

   Lead nitrate solution reacts with potassium iodide solution (Activity 1.2) by exchanging ions, giving a yellow precipitate of lead iodide and potassium nitrate solution: $\text{Pb}({\text{NO}}_3{)}_2(aq)+2\text{KI}(aq)\to {\text{PbI}}_2(s)+2{\text{KNO}}_3(aq)$. Lead and potassium swap their partners, so this is a double displacement reaction. (Another example: ${\text{AgNO}}_3(aq)+\text{NaCl}(aq)\to \text{AgCl}(s)+{\text{NaNO}}_3(aq)$.)

   Step-by-step solution →
8. Q28 NCERT · Intext Questions (Page 13)
   Identify ​the substances that ​are oxidised and the substances that are ​reduced in ​the following reactions.

   1. (i)$4\text{Na}(s)+{\text{O}}_2(g)\to 2{\text{Na}}_2\text{O}(s)$
   2. (ii)$\text{CuO}(s)+{\text{H}}_2(g)\to \text{Cu}(s)+{\text{H}}_2\text{O}(l)$
   Show answer

   (i) $\text{Na}$ is oxidised and ${\text{O}}_2$ is reduced; (ii) ${\text{H}}_2$ is oxidised and $\text{CuO}$ is reduced.

   (i) Sodium ($\text{Na}$) is oxidised to sodium oxide because it gains oxygen; oxygen (${\text{O}}_2$) is reduced. (ii) Hydrogen (${\text{H}}_2$) is oxidised to water because it gains oxygen; copper oxide ($\text{CuO}$) is reduced to copper because it loses oxygen.

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9. Q29 Exemplar · Multiple Choice Questions
   Which of ​the following ​is not ​a ​physical change?

   1. (a)Boiling of water to give water vapour
   2. (b)Melting of ice to give water
   3. (c)Dissolution of salt in water
   4. (d)Combustion of Liquefied Petroleum Gas (LPG)
   Show answer

   (d) Combustion of Liquefied Petroleum Gas (LPG)

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10. Q30 Exemplar · Multiple Choice Questions
    The following reaction ​is an example of a $$4{\text{NH}}_3(g)+5{\text{O}}_2(g)\to 4\text{NO}(g)+6{\text{H}}_2\text{O}(g)$$

    1. (i)displacement reaction
    2. (ii)combination reaction
    3. (iii)redox reaction
    4. (iv)​neutralisation ​reaction

    1. (a)(i) and (iv)
    2. (b)(ii) and (iii)
    3. (c)(i) and (iii)
    4. (d)(iii) and (iv)
    Show answer

    (c) (i) and (iii)

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11. Q31 Exemplar · Multiple Choice Questions
    Which of the following statements about the ​given reaction are correct? $$3\text{Fe}(s)+4{\text{H}}_2\text{O}(g)\to {\text{Fe}}_3{\text{O}}_4(s)+4{\text{H}}_2(g)$$

    1. (i)Iron metal is getting oxidised
    2. (ii)​Water is getting reduced
    3. (iii)Water is ​acting as reducing agent
    4. (iv)​Water is acting ​as oxidising ​agent

    1. (a)(i), (ii) and (iii)
    2. (b)(iii) and (iv)
    3. (c)(i), (ii) and (iv)
    4. (d)(ii) and (iv)
    Show answer

    (c) (i), (ii) and (iv)

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12. Q32 Exemplar · Multiple Choice Questions
    Which of the following ​are exothermic ​processes?

    1. (i)Reaction of water ​with quick lime
    2. (ii)Dilution ​of ​an ​acid
    3. (iii)Evaporation of ​water
    4. (iv)Sublimation of camphor (crystals)

    1. (a)(i) and (ii)
    2. (b)(ii) and (iii)
    3. (c)(i) and (iv)
    4. (d)(iii) and (iv)
    Show answer

    (a) (i) and (ii)

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13. Q33 Exemplar · Multiple Choice Questions
    Three beakers labelled as A, B and C each containing 25 mL of water were taken. A small ​amount of NaOH, anhydrous ${\text{CuSO}}_4$ and NaCl were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, ​whereas in case of beaker C, ​the temperature of the solution falls. Which one of the ​following statement(s) is(are) correct?

    1. (i)In beakers A and B, exothermic process has occurred.
    2. (ii)In beakers A and B, ​endothermic process has occurred.
    3. (iii)In beaker C exothermic process ​has occurred.
    4. (iv)In ​beaker C endothermic process has occurred.

    1. (a)(i) only
    2. (b)(ii) only
    3. (c)(i) and (iv)
    4. (d)(ii) and (iii)
    Show answer

    (c) (i) and (iv)

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14. Q34 Exemplar · Multiple Choice Questions
    A dilute ferrous sulphate solution ​was gradually added ​to the ​beaker containing acidified permanganate solution. The light purple colour of the solution ​fades and finally disappears. Which of the following ​is the correct explanation for ​the observation?

    1. (a)${\text{KMnO}}_4$ is an oxidising agent, it oxidises ${\text{FeSO}}_4$
    2. (b)${\text{FeSO}}_4$ acts as an oxidising agent and oxidises ${\text{KMnO}}_4$
    3. (c)The colour disappears due to dilution; no reaction is involved
    4. (d)${\text{KMnO}}_4$ is an unstable compound and decomposes in presence of ${\text{FeSO}}_4$ to a colourless compound.
    Show answer

    (a) ${\text{KMnO}}_4$ is an oxidising agent, it oxidises ${\text{FeSO}}_4$

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15. Q35 Exemplar · Multiple Choice Questions
    Which ​among the ​following is(are) ​double ​displacement ​reaction(s)?

    1. (i)$\text{Pb}+{\text{CuCl}}_2\to {\text{PbCl}}_2+\text{Cu}$
    2. (ii)${\text{Na}}_2{\text{SO}}_4+{\text{BaCl}}_2\to {\text{BaSO}}_4+2\text{NaCl}$
    3. (iii)$\text{C}+{\text{O}}_2\to {\text{CO}}_2$
    4. (iv)${\text{CH}}_4+2{\text{O}}_2\to {\text{CO}}_2+2{\text{H}}_2\text{O}$

    1. (a)(i) and (iv)
    2. (b)(ii) only
    3. (c)(i) and (ii)
    4. (d)(iii) and (iv)
    Show answer

    (b) (ii) only

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16. Q36 Exemplar · Multiple Choice Questions
    Which among the following statement(s) is(are) true? Exposure ​of silver chloride to sunlight for a long duration ​turns grey due ​to

    1. (i)the ​formation of silver by decomposition of silver chloride
    2. (ii)sublimation of ​silver chloride
    3. (iii)decomposition of ​chlorine gas from silver chloride
    4. (iv)oxidation ​of silver ​chloride

    1. (a)(i) only
    2. (b)(i) and (iii)
    3. (c)(ii) and (iii)
    4. (d)(iv) only
    Show answer

    (a) (i) only

    Step-by-step solution →
17. Q37 Exemplar · Multiple Choice Questions
    Solid ​calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. This ​process is called slaking of lime. Calcium hydroxide dissolves in water to form its solution ​called lime water. Which among the following is (are) true about slaking of lime and the solution formed?

    1. (i)It ​is an endothermic reaction
    2. (ii)It is an exothermic reaction
    3. (iii)The pH of the resulting solution will be more than ​seven
    4. (iv)The pH of the resulting solution will be ​less ​than seven

    1. (a)(i) and (ii)
    2. (b)(ii) and (iii)
    3. (c)(i) and (iv)
    4. (d)(iii) and (iv)
    Show answer

    (b) (ii) and (iii)

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18. Q38 Exemplar · Multiple Choice Questions
    Barium ​chloride on ​reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of ​the ​reaction involved?

    1. (i)​Displacement reaction
    2. (ii)Precipitation ​reaction
    3. (iii)Combination ​reaction
    4. (iv)Double displacement reaction

    1. (a)(i) only
    2. (b)(ii) only
    3. (c)(iv) only
    4. (d)(ii) and (iv)
    Show answer

    (d) (ii) and (iv)

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19. Q39 Exemplar · Multiple Choice Questions
    Electrolysis of ​water is ​a decomposition ​reaction. The ​mole ratio of hydrogen and oxygen gases liberated ​during electrolysis ​of water is

    1. (a)$1:1$
    2. (b)$2:1$
    3. (c)$4:1$
    4. (d)$1:2$
    Show answer

    (b) $2:1$

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20. Q40 Exemplar · Multiple Choice Questions
    Which of ​the following is(are) ​an endothermic process(es)?

    1. (i)Dilution of sulphuric ​acid
    2. (ii)​Sublimation of ​dry ice
    3. (iii)Condensation of water vapours
    4. (iv)​Evaporation of ​water

    1. (a)(i) and (iii)
    2. (b)(ii) only
    3. (c)(iii) only
    4. (d)(ii) and (iv)
    Show answer

    (d) (ii) and (iv)

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