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 3 of 11

1. Q41 Exemplar · Multiple Choice Questions
   In the double displacement ​reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of ​lead iodide is formed. While performing the activity if lead ​nitrate is not available, ​which ​of the ​following ​can be used in place of lead nitrate?

   1. (a)Lead sulphate (insoluble)
   2. (b)Lead acetate
   3. (c)Ammonium nitrate
   4. (d)Potassium sulphate
   Show answer

   (b) Lead acetate

   Step-by-step solution →
2. Q42 Exemplar · Multiple Choice Questions
   Which of the ​following gases ​can be used ​for ​storage of fresh sample ​of an oil for ​a long ​time?

   1. (a)Carbon dioxide or oxygen
   2. (b)Nitrogen or oxygen
   3. (c)Carbon dioxide or helium
   4. (d)Helium or nitrogen
   Show answer

   (d) Helium or nitrogen

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3. Q43 Exemplar · Multiple Choice Questions
   The following reaction is used ​for the preparation ​of oxygen gas ​in the laboratory $$2{\text{KClO}}_3(s)\underset{\text{Catalyst}}{\overset{\text{Heat}}{\to }}2\text{KCl}(s)+3{\text{O}}_2(g)$$ Which of the following ​statement(s) is(are) correct about ​the ​reaction?

   1. (a)It is a decomposition reaction and endothermic in nature
   2. (b)It is a combination reaction
   3. (c)It is a decomposition reaction and accompanied by release of heat
   4. (d)It is a photochemical decomposition reaction and exothermic in nature
   Show answer

   (a) It is a decomposition reaction and endothermic in nature

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4. Q44 Exemplar · Multiple Choice Questions
   Which ​one ​of the ​following ​processes ​involve chemical reactions?

   1. (a)Storing of oxygen gas under pressure in a gas cylinder
   2. (b)Liquefaction of air
   3. (c)Keeping petrol in a china dish in the open
   4. (d)Heating copper wire in presence of air at high temperature
   Show answer

   (d) Heating copper wire in presence of air at high temperature

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5. Q45 Exemplar · Multiple Choice Questions
   In ​which of the ​following chemical ​equations, ​the abbreviations represent the correct ​states of the reactants ​and ​products involved at reaction temperature?

   1. (a)$2{\text{H}}_2(l)+{\text{O}}_2(l)\to 2{\text{H}}_2\text{O}(g)$
   2. (b)$2{\text{H}}_2(g)+{\text{O}}_2(l)\to 2{\text{H}}_2\text{O}(l)$
   3. (c)$2{\text{H}}_2(g)+{\text{O}}_2(g)\to 2{\text{H}}_2\text{O}(l)$
   4. (d)$2{\text{H}}_2(g)+{\text{O}}_2(g)\to 2{\text{H}}_2\text{O}(g)$
   Show answer

   (d) $2{\text{H}}_2(g)+{\text{O}}_2(g)\to 2{\text{H}}_2\text{O}(g)$

   Step-by-step solution →
6. Q46 Exemplar · Multiple Choice Questions
   Which ​of the ​following are ​combination reactions?

   1. (i)$2{\text{KClO}}_3\stackrel{\text{Heat}}{\to }2\text{KCl}+3{\text{O}}_2$
   2. (ii)$\text{MgO}+{\text{H}}_2\text{O}\to {\text{Mg(OH)}}_2$
   3. (iii)$4\text{Al}+3{\text{O}}_2\to 2{\text{Al}}_2{\text{O}}_3$
   4. (iv)$\text{Zn}+{\text{FeSO}}_4\to {\text{ZnSO}}_4+\text{Fe}$

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

   (d) (ii) and (iii)

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7. Q47 Exemplar · Short Answer Questions
   Write the balanced chemical ​equations for the following reactions and identify the type of ​reaction in each case.

   1. (a)Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas.
   2. (b)Sodium ​hydroxide solution is treated with acetic acid to form sodium acetate and water.
   3. (c)​Ethanol is warmed with ethanoic ​acid to form ethyl acetate in the presence of concentrated ${\text{H}}_2{\text{SO}}_4$.
   4. (d)Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases ​heat ​and light.
   Show answer

   The reactions are a combination, a neutralisation (double displacement), an esterification (double displacement) and a combustion.

   (a) ${\text{N}}_2(g)+3{\text{H}}_2(g)\underset{\text{catalyst}}{\overset{773\text{K}}{\to }}2{\text{NH}}_3(g)$ — combination reaction. (b) $\text{NaOH}(aq)+{\text{CH}}_3\text{COOH}(aq)\to {\text{CH}}_3\text{COONa}(aq)+{\text{H}}_2\text{O}(l)$ — double displacement, specifically a neutralisation. (c) ${\text{CH}}_3\text{COOH}+{\text{C}}_2{\text{H}}_5\text{OH}\stackrel{\text{conc. }{\text{H}}_2{\text{SO}}_4}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$ — esterification, a double displacement. (d) ${\text{C}}_2{\text{H}}_4(g)+3{\text{O}}_2(g)\to 2{\text{CO}}_2(g)+2{\text{H}}_2\text{O}(g)+\text{heat}+\text{light}$ — combustion, an oxidation reaction and exothermic.

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8. Q48 Exemplar · Short Answer Questions
   Write the ​balanced chemical equations for ​the following reactions and identify the type of reaction ​in each ​case.

   1. (a)Thermit reaction, iron (III) oxide reacts with aluminium and ​gives molten iron and aluminium oxide.
   2. (b)Magnesium ribbon is burnt in an atmosphere of nitrogen ​gas ​to form solid magnesium nitride.
   3. (c)Chlorine gas is passed in an aqueous ​potassium iodide solution to form potassium chloride solution and solid iodine.
   4. (d)Ethanol is burnt in air to form carbon dioxide, water and releases heat.
   Show answer

   The reactions are displacement (a), combination (b), displacement (c) and combustion (d), each with its balanced equation.

   (a) ${\text{Fe}}_2{\text{O}}_3(s)+2\text{Al}(s)\to 2\text{Fe}(l)+{\text{Al}}_2{\text{O}}_3(s)+\text{heat}$ — displacement reaction, also a redox reaction and strongly exothermic. (b) $3\text{Mg}(s)+{\text{N}}_2(g)\to {\text{Mg}}_3{\text{N}}_2(s)$ — combination reaction. (c) ${\text{Cl}}_2(g)+2\text{KI}(aq)\to 2\text{KCl}(aq)+{\text{I}}_2(s)$ — displacement reaction. (d) ${\text{C}}_2{\text{H}}_5\text{OH}(l)+3{\text{O}}_2(g)\to 2{\text{CO}}_2(g)+3{\text{H}}_2\text{O}(g)+\text{heat}$ — combustion, an oxidation reaction and exothermic.

   Step-by-step solution →
9. Q49 Exemplar · Short Answer Questions
   Complete the ​missing components/variables ​given ​as $x$ ​and $y$ ​in the ​following ​reactions

   1. (a)${\text{Pb(NO}}_3{)}_2(aq)+2\text{KI}(aq)\to {\text{PbI}}_2(x)+2{\text{KNO}}_3(y)$
   2. (b)$\text{Cu}(s)+2{\text{AgNO}}_3(aq)\to {\text{Cu(NO}}_3{)}_2(aq)+x(s)$
   3. (c)$\text{Zn}(s)+{\text{H}}_2{\text{SO}}_4(aq)\to {\text{ZnSO}}_4(x)+{\text{H}}_2(y)$
   4. (d)${\text{CaCO}}_3(s)\stackrel{x}{\to }\text{CaO}(s)+{\text{CO}}_2(g)$
   Show answer

   The missing items are (a) solid and aqueous, (b) silver, (c) aqueous and gas, (d) heat.

   (a) $x=(s)$ and $y=(aq)$, since lead iodide is the insoluble yellow precipitate while potassium nitrate stays dissolved. (b) $x=\text{Ag}$, so the equation reads $\text{Cu}(s)+2{\text{AgNO}}_3(aq)\to {\text{Cu(NO}}_3{)}_2(aq)+2\text{Ag}(s)$; copper displaces the less reactive silver. (c) $x=(aq)$ and $y=(g)$, since zinc sulphate remains in solution while hydrogen escapes as a gas. (d) $x=\text{heat}$, because calcium carbonate decomposes only on strong heating, at about $1200\text{K}$.

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10. Q50 Exemplar · Short Answer Questions
    Which among the following changes are exothermic ​or endothermic ​in nature?

    1. (a)Decomposition ​of ferrous ​sulphate
    2. (b)Dilution of sulphuric acid
    3. (c)​Dissolution of sodium hydroxide ​in ​water
    4. (d)Dissolution of ammonium chloride in ​water
    Show answer

    Decomposition of ferrous sulphate and dissolution of ammonium chloride are endothermic; dilution of sulphuric acid and dissolution of sodium hydroxide are exothermic.

    (a) Endothermic — ferrous sulphate decomposes only while it is being heated, $2{\text{FeSO}}_4(s)\stackrel{\text{heat}}{\to }{\text{Fe}}_2{\text{O}}_3(s)+{\text{SO}}_2(g)+{\text{SO}}_3(g)$, so energy is absorbed. (b) Exothermic — diluting sulphuric acid releases a large amount of heat and the vessel becomes hot. (c) Exothermic — dissolving sodium hydroxide releases more energy on hydration than the lattice costs, so the solution warms. (d) Endothermic — dissolving ammonium chloride costs more energy than hydration returns, so heat is drawn from the water and the solution cools.

    Step-by-step solution →
11. Q51 Exemplar · Short Answer Questions
    Identify the ​reducing ​agent ​in ​the following ​reactions

    1. (a)$4{\text{NH}}_3+5{\text{O}}_2\to 4\text{NO}+6{\text{H}}_2\text{O}$
    2. (b)${\text{H}}_2\text{O}+{\text{F}}_2\to \text{HF}+\text{HOF}$
    3. (c)${\text{Fe}}_2{\text{O}}_3+3\text{CO}\to 2\text{Fe}+3{\text{CO}}_2$
    4. (d)$2{\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}$
    Show answer

    The reducing agents are ammonia, water, carbon monoxide and hydrogen; each is itself oxidised.

    (a) Ammonia, ${\text{NH}}_3$; (b) Water, ${\text{H}}_2\text{O}$; (c) Carbon monoxide, $\text{CO}$; (d) Hydrogen, ${\text{H}}_2$

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12. Q52 Exemplar · Short Answer Questions
    Identify ​the oxidising ​agent ​(oxidant) in ​the ​following ​reactions

    1. (a)${\text{Pb}}_3{\text{O}}_4+8\text{HCl}\to 3{\text{PbCl}}_2+{\text{Cl}}_2+4{\text{H}}_2\text{O}$
    2. (b)$2\text{Mg}+{\text{O}}_2\to 2\text{MgO}$
    3. (c)${\text{CuSO}}_4+\text{Zn}\to \text{Cu}+{\text{ZnSO}}_4$
    4. (d)${\text{V}}_2{\text{O}}_5+5\text{Ca}\to 2\text{V}+5\text{CaO}$
    5. (e)$3\text{Fe}+4{\text{H}}_2\text{O}\to {\text{Fe}}_3{\text{O}}_4+4{\text{H}}_2$
    6. (f)$\text{CuO}+{\text{H}}_2\to \text{Cu}+{\text{H}}_2\text{O}$
    Show answer

    The oxidising agents in (a) to (f) are ${\text{Pb}}_3{\text{O}}_4$, ${\text{O}}_2$, ${\text{CuSO}}_4$, ${\text{V}}_2{\text{O}}_5$, ${\text{H}}_2\text{O}$ and $\text{CuO}$; each is reduced.

    (a) Red lead, ${\text{Pb}}_3{\text{O}}_4$; (b) Oxygen, ${\text{O}}_2$; (c) Copper sulphate, ${\text{CuSO}}_4$; (d) Vanadium(V) oxide, ${\text{V}}_2{\text{O}}_5$; (e) Water (steam), ${\text{H}}_2\text{O}$; (f) Copper(II) oxide, $\text{CuO}$

    Step-by-step solution →
13. Q53 Exemplar · Short Answer Questions
    Write the balanced chemical equations for the following reactions

    1. (a)​Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations ​gives ​sodium chloride ​and ​sodium hydrogencarbonate.
    2. (b)Sodium hydrogencarbonate on reaction with hydrochloric acid gives sodium ​chloride, water and liberates carbon dioxide.
    3. (c)Copper sulphate on treatment with ​potassium iodide ​precipitates cuprous iodide (${\text{Cu}}_2{\text{I}}_2$), liberates iodine gas and also forms potassium sulphate.
    Show answer

    The balanced equations are ${\text{Na}}_2{\text{CO}}_3+\text{HCl}\to \text{NaCl}+{\text{NaHCO}}_3$, ${\text{NaHCO}}_3+\text{HCl}\to \text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$ and $2{\text{CuSO}}_4+4\text{KI}\to {\text{Cu}}_2{\text{I}}_2+{\text{I}}_2+2{\text{K}}_2{\text{SO}}_4$.

    (a) ${\text{Na}}_2{\text{CO}}_3(aq)+\text{HCl}(aq)\to \text{NaCl}(aq)+{\text{NaHCO}}_3(aq)$; (b) ${\text{NaHCO}}_3(aq)+\text{HCl}(aq)\to \text{NaCl}(aq)+{\text{H}}_2\text{O}(l)+{\text{CO}}_2(g)$; (c) $2{\text{CuSO}}_4(aq)+4\text{KI}(aq)\to {\text{Cu}}_2{\text{I}}_2(s)+{\text{I}}_2+2{\text{K}}_2{\text{SO}}_4(aq)$

    Step-by-step solution →
14. Q54 Exemplar · Short Answer Questions
    A ​solution of potassium chloride when mixed with ​silver nitrate solution, ​an insoluble white ​substance is formed. Write ​the chemical reaction involved ​and also ​mention the type of the chemical reaction?
    Show answer

    Potassium chloride and silver nitrate give white silver chloride and potassium nitrate: a double displacement (precipitation) reaction.

    $\text{KCl}(aq)+{\text{AgNO}}_3(aq)\to \text{AgCl}(s)+{\text{KNO}}_3(aq)$. The insoluble white substance is silver chloride, $\text{AgCl}$. The reaction is a double displacement reaction, and since an insoluble solid separates out it is also a precipitation reaction.

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15. Q55 Exemplar · Short Answer Questions
    Ferrous sulphate decomposes with the evolution of ​a gas having ​a characteristic ​odour of burning ​sulphur. Write the ​chemical ​reaction involved and identify ​the type of reaction.
    Show answer

    Heating ferrous sulphate gives ferric oxide, sulphur dioxide and sulphur trioxide: a thermal decomposition reaction.

    $2{\text{FeSO}}_4(s)\stackrel{\text{Heat}}{\to }{\text{Fe}}_2{\text{O}}_3(s)+{\text{SO}}_2(g)+{\text{SO}}_3(g)$. The gas smelling of burning sulphur is sulphur dioxide. The reaction is a decomposition reaction, specifically a thermal decomposition, and it is endothermic. If the green crystals ${\text{FeSO}}_4\cdot 7{\text{H}}_2\text{O}$ are used, they first lose their water of crystallisation: ${\text{FeSO}}_4\cdot 7{\text{H}}_2\text{O}(s)\stackrel{\text{Heat}}{\to }{\text{FeSO}}_4(s)+7{\text{H}}_2\text{O}(g)$.

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16. Q56 Exemplar · Short Answer Questions
    Why ​do ​fire flies ​glow at night?
    Show answer

    A protein pigment in the abdomen is oxidised by air with an enzyme, and the energy is released as light (bioluminescence).

    Fire flies glow because of a chemical reaction going on inside them. Their abdomen contains a protein pigment which is oxidised by the oxygen of the air in the presence of an enzyme, and the energy released by this oxidation is given out as visible light rather than as heat. It is therefore an oxidation reaction accompanied by the emission of light — chemiluminescence, or, since it takes place in a living organism, bioluminescence. Because the reaction needs oxygen, the insect can switch the glow on and off by controlling the air reaching those cells.

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17. Q57 Exemplar · Short Answer Questions
    Grapes hanging on the plant ​do not ​ferment but after being plucked from the plant can be fermented. ​Under ​what ​conditions do ​these grapes ferment? Is it ​a chemical or ​a physical change?
    Show answer

    Plucked grapes ferment given yeast, sugar, moisture, moderate warmth and no air; it is a chemical change, since new substances form.

    Grapes ferment once they have been plucked, because the fruit is then no longer protected by the living plant and the micro-organisms that live on the skin can reach the sugary juice. The conditions needed are the presence of yeast, a supply of sugar and moisture, a moderately warm temperature, and the absence of air. Under these conditions the glucose is broken down: ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6\stackrel{\text{yeast}}{\to }2{\text{C}}_2{\text{H}}_5\text{OH}+2{\text{CO}}_2$. It is a chemical change, because new substances (ethanol and carbon dioxide) are formed and the grape juice cannot be recovered from them.

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18. Q58 Exemplar · Short Answer Questions
    Which ​among the following ​are physical or chemical ​changes?

    1. (a)Evaporation of petrol
    2. (b)Burning ​of Liquefied Petroleum Gas ​(LPG)
    3. (c)Heating of an iron rod ​to red hot.
    4. (d)Curdling of ​milk
    5. (e)Sublimation of solid ammonium chloride
    Show answer

    Evaporation of petrol, heating an iron rod and sublimation of ammonium chloride are physical changes; burning LPG and curdling of milk are chemical.

    (a) Physical change — only the state of the petrol alters, and condensing the vapour gives the petrol back; (b) Chemical change — the fuel is converted into carbon dioxide and water with the release of heat and light, and cannot be recovered; (c) Physical change — the rod glows because it is hot, but it is still iron and returns to normal on cooling; (d) Chemical change — bacteria turn the lactose into lactic acid and the milk sets as curd, which cannot be turned back into milk; (e) Physical change — sublimation is a change of state, and the ammonium chloride re-forms unchanged on cooling.

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19. Q59 Exemplar · Short Answer Questions
    During the reaction ​of some metals with dilute ​hydrochloric acid, following observations were made.

    1. (a)Silver metal does not show ​any change
    2. (b)The temperature of the reaction mixture rises when aluminium (Al) is added.
    3. (c)The ​reaction ​of sodium ​metal is found to be highly explosive
    4. (d)Some bubbles of a gas are seen ​when lead (Pb) is reacted with the acid. Explain these observations giving suitable ​reasons.
    Show answer

    Silver is below hydrogen and does not react; aluminium, sodium and lead are above it and displace hydrogen at different rates.

    (a) Silver lies below hydrogen in the reactivity series, so it cannot displace hydrogen from dilute hydrochloric acid and no reaction occurs; (b) Aluminium lies well above hydrogen and displaces it readily, and since the reaction is exothermic the temperature of the mixture rises: $2\text{Al}(s)+6\text{HCl}(aq)\to 2{\text{AlCl}}_3(aq)+3{\text{H}}_2(g)$; (c) Sodium lies far above hydrogen, so the reaction is both very fast and strongly exothermic — so much heat is released so quickly that the hydrogen produced catches fire and the reaction becomes explosive: $2\text{Na}(s)+2\text{HCl}(aq)\to 2\text{NaCl}(aq)+{\text{H}}_2(g)$; (d) Lead lies only just above hydrogen, so it does displace hydrogen but slowly, and the sparingly soluble lead chloride formed coats the metal and slows it further, so only a few bubbles are seen: $\text{Pb}(s)+2\text{HCl}(aq)\to {\text{PbCl}}_2(s)+{\text{H}}_2(g)$.

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20. Q60 Exemplar · Short Answer Questions
    A substance X, which is an oxide of a group 2 ​element, is ​used ​intensively in the cement industry. This element is present in bones ​also. On treatment with water it forms ​a solution which turns red litmus blue. Identify X ​and also write the chemical reactions involved.
    Show answer

    X is calcium oxide ($\text{CaO}$); with water it forms calcium hydroxide, a base that turns red litmus blue.

    X is calcium oxide, $\text{CaO}$, commonly called quicklime. Calcium is a group 2 element, calcium compounds make up the mineral part of bone, and lime is used in vast quantity in the cement industry. With water it combines to give calcium hydroxide, releasing much heat: $\text{CaO}(s)+{\text{H}}_2\text{O}(l)\to {\text{Ca(OH)}}_2(aq)+\text{heat}$ — a combination reaction, and exothermic. The calcium hydroxide gives hydroxide ions in solution, ${\text{Ca(OH)}}_2(aq)\to {\text{Ca}}^{2+}(aq)+2{\text{OH}}^{-}(aq)$, which is why the solution is basic and turns red litmus blue.

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