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 4 of 11

1. Q61 Exemplar · Short Answer Questions
   Write a balanced ​chemical equation for each of the following ​reactions and also classify ​them.

   1. (a)Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.
   2. (b)A ​piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.
   3. (c)Iron (III) oxide on ​heating with carbon monoxide gas reacts to form ​solid iron and liberates carbon dioxide gas.
   4. (d)Hydrogen sulphide gas reacts ​with oxygen gas to form solid sulphur and ​liquid water.
   Show answer

   The reactions are classified as double displacement (a), displacement (b) and redox (c) and (d), each with its balanced equation.

   (a) ${\text{Pb(CH}}_3{\text{COO)}}_2(aq)+2\text{HCl}(aq)\to {\text{PbCl}}_2(s)+2{\text{CH}}_3\text{COOH}(aq)$ — a double displacement reaction; (b) $2\text{Na}(s)+2{\text{C}}_2{\text{H}}_5\text{OH}(l)\to 2{\text{C}}_2{\text{H}}_5\text{ONa}+{\text{H}}_2(g)$ — a displacement reaction; (c) ${\text{Fe}}_2{\text{O}}_3(s)+3\text{CO}(g)\stackrel{\text{Heat}}{\to }2\text{Fe}(s)+3{\text{CO}}_2(g)$ — a redox reaction, in which the iron(III) oxide is reduced and the carbon monoxide oxidised; (d) $2{\text{H}}_2\text{S}(g)+{\text{O}}_2(g)\to 2\text{S}(s)+2{\text{H}}_2\text{O}(l)$ — a redox reaction, the oxidation of hydrogen sulphide.

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2. Q62 Exemplar · Short Answer Questions
   Why do ​we store silver chloride ​in ​dark ​coloured ​bottles?
   Show answer

   Silver chloride decomposes in light into silver and chlorine; a dark bottle absorbs the light and prevents this.

   Silver chloride decomposes in sunlight into silver and chlorine, $2\text{AgCl}(s)\stackrel{\text{sunlight}}{\to }2\text{Ag}(s)+{\text{Cl}}_2(g)$, which is why a white sample left in the light slowly turns grey — the grey being finely divided metallic silver. This is a photochemical decomposition reaction, and light is what supplies the energy for it. A dark coloured bottle absorbs the light before it can reach the solid, so the decomposition cannot take place and the silver chloride keeps its composition.

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3. Q63 Exemplar · Short Answer Questions
   Balance ​the ​following ​chemical equations and ​identify the type ​of chemical ​reaction.

   1. (a)$\text{Mg}(s)+{\text{Cl}}_2(g)\to {\text{MgCl}}_2(s)$
   2. (b)$\text{HgO}(s)\stackrel{\text{Heat}}{\to }\text{Hg}(l)+{\text{O}}_2(g)$
   3. (c)$\text{Na}(s)+\text{S}(s)\stackrel{\text{Fuse}}{\to }{\text{Na}}_2\text{S}(s)$
   4. (d)${\text{TiCl}}_4(l)+\text{Mg}(s)\to \text{Ti}(s)+{\text{MgCl}}_2(s)$
   5. (e)$\text{CaO}(s)+{\text{SiO}}_2(s)\to {\text{CaSiO}}_3(s)$
   6. (f)${\text{H}}_2{\text{O}}_2(l)\stackrel{\text{UV}}{\to }{\text{H}}_2\text{O}(l)+{\text{O}}_2(g)$
   Show answer

   Reactions (a), (c) and (e) are combination, (b) and (f) are decomposition, and (d) is displacement.

   (a) $\text{Mg}(s)+{\text{Cl}}_2(g)\to {\text{MgCl}}_2(s)$ — already balanced; a combination reaction; (b) $2\text{HgO}(s)\stackrel{\text{Heat}}{\to }2\text{Hg}(l)+{\text{O}}_2(g)$ — a decomposition reaction, thermal decomposition; (c) $2\text{Na}(s)+\text{S}(s)\stackrel{\text{Fuse}}{\to }{\text{Na}}_2\text{S}(s)$ — a combination reaction; (d) ${\text{TiCl}}_4(l)+2\text{Mg}(s)\to \text{Ti}(s)+2{\text{MgCl}}_2(s)$ — a displacement reaction; (e) $\text{CaO}(s)+{\text{SiO}}_2(s)\to {\text{CaSiO}}_3(s)$ — already balanced; a combination reaction; (f) $2{\text{H}}_2{\text{O}}_2(l)\stackrel{\text{UV}}{\to }2{\text{H}}_2\text{O}(l)+{\text{O}}_2(g)$ — a decomposition reaction, photochemical decomposition.

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4. Q64 Exemplar · Short Answer Questions
   A magnesium ribbon is ​burnt ​in oxygen to give a white ​compound X accompanied by emission of light. ​If the burning ribbon is now placed in ​an ​atmosphere of nitrogen, it continues to burn and forms a compound Y.

   1. (a)​Write the chemical formulae of X and Y.
   2. (b)Write a balanced ​chemical equation, when X is dissolved in water.
   Show answer

   X is $\text{MgO}$ and Y is ${\text{Mg}}_3{\text{N}}_2$; $\text{MgO}$ dissolves in water to give magnesium hydroxide, ${\text{Mg(OH)}}_2$.

   (a) $X=\text{MgO}$, magnesium oxide, the white compound formed in oxygen; $Y={\text{Mg}}_3{\text{N}}_2$, magnesium nitride, formed when the burning ribbon is placed in nitrogen. (b) $\text{MgO}(s)+{\text{H}}_2\text{O}(l)\to {\text{Mg(OH)}}_2(aq)$ — magnesium oxide dissolves in water to give magnesium hydroxide, a combination reaction whose product makes the water faintly basic.

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5. Q65 Exemplar · Short Answer Questions
   Zinc ​liberates ​hydrogen ​gas ​when reacted with dilute hydrochloric acid, whereas copper ​does ​not. Explain why?
   Show answer

   Zinc lies above hydrogen and displaces it from the acid; copper lies below hydrogen, so it cannot and no gas is liberated.

   Zinc lies above hydrogen in the reactivity series, so it gives up its electrons more readily than hydrogen does. It can therefore hand them to the hydrogen ions of the acid, taking their place in the salt and setting hydrogen gas free: $\text{Zn}(s)+2\text{HCl}(aq)\to {\text{ZnCl}}_2(aq)+{\text{H}}_2(g)$. Copper lies below hydrogen in the series, so it holds its electrons more tightly than hydrogen does and cannot give them to the hydrogen ions. No electron transfer is possible, so copper does not displace hydrogen from dilute hydrochloric acid and no gas is liberated.

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6. Q66 Exemplar · Short Answer Questions
   A silver article generally turns black when kept in the open for ​a few ​days. The article when rubbed with ​toothpaste again starts ​shining.

   1. (a)Why do silver articles turn black when kept in the open ​for a few days? Name the phenomenon involved.
   2. (b)Name ​the black ​substance formed and give its ​chemical formula.
   Show answer

   Silver reacts with hydrogen sulphide of the air to form black silver sulphide, ${\text{Ag}}_2\text{S}$, a corrosion called tarnishing.

   (a) Silver articles turn black because the silver at the surface is slowly attacked by the small amount of hydrogen sulphide present in the air, in the presence of oxygen, and becomes coated with a thin layer of a new compound: $4\text{Ag}(s)+2{\text{H}}_2\text{S}(g)+{\text{O}}_2(g)\to 2{\text{Ag}}_2\text{S}(s)+2{\text{H}}_2\text{O}(l)$. The phenomenon is corrosion — for silver it is usually called tarnishing. (b) The black substance is silver sulphide, formula ${\text{Ag}}_2\text{S}$. Rubbing with toothpaste, which is a mild abrasive, scrapes this thin layer away and exposes the bright silver beneath.

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7. Q67 Exemplar · Long Answer Questions
   On heating blue coloured powder ​of ​copper (II) nitrate in a boiling tube, ​copper oxide (black), oxygen gas ​and a brown ​gas X is formed

   1. (a)Write a balanced ​chemical ​equation of the reaction.
   2. (b)Identity the brown gas X evolved.
   3. (c)Identity the ​type of reaction.
   4. (d)What could be the pH range of aqueous solution of the gas X?
   Show answer

   Gas X is ${\text{NO}}_2$, formed by thermal decomposition of copper(II) nitrate; its aqueous solution is acidic, with pH below $7$.

   (a) $2{\text{Cu(NO}}_3{)}_2(s)\stackrel{\text{Heat}}{\to }2\text{CuO}(s)+4{\text{NO}}_2(g)+{\text{O}}_2(g)$; (b) The brown gas X is nitrogen dioxide, ${\text{NO}}_2$ — the only brown gas the elements present can give; (c) It is a decomposition reaction, specifically a thermal decomposition, and it is endothermic; within the nitrate ion it is also a redox change, since nitrogen falls from $+5$ to $+4$ while some oxygen rises from $-2$ to $0$; (d) Nitrogen dioxide dissolves in water to give nitrous and nitric acid, $2{\text{NO}}_2+{\text{H}}_2\text{O}\to {\text{HNO}}_2+{\text{HNO}}_3$, so the solution is acidic and its pH lies below $7$, in the range $0$ to $7$ — and because nitric acid is a strong acid the value falls to the low single figures.

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8. Q68 Exemplar · Long Answer Questions
   Give ​the ​characteristic ​tests for ​the following ​gases

   1. (a)${\text{CO}}_2$
   2. (b)${\text{SO}}_2$
   3. (c)${\text{O}}_2$
   4. (d)${\text{H}}_2$
   Show answer

   ${\text{CO}}_2$ turns lime water milky; ${\text{SO}}_2$ turns acidified dichromate paper orange to green; ${\text{O}}_2$ relights a glowing splint; ${\text{H}}_2$ burns with a pop.

   (a) ${\text{CO}}_2$: pass the gas through freshly prepared lime water — it turns milky, because insoluble calcium carbonate is formed, ${\text{Ca(OH)}}_2(aq)+{\text{CO}}_2(g)\to {\text{CaCO}}_3(s)+{\text{H}}_2\text{O}(l)$ (the milkiness disappears if excess gas is passed, as the soluble hydrogencarbonate forms); (b) ${\text{SO}}_2$: pass the gas over paper moistened with acidified potassium dichromate — the orange colour turns green, because sulphur dioxide is a reducing agent and reduces the dichromate to a chromium(III) salt; the gas also smells of burning sulphur and turns moist blue litmus red; (c) ${\text{O}}_2$: bring a glowing splint to the mouth of the tube — it relights and burns brightly, because oxygen supports combustion without itself burning; (d) ${\text{H}}_2$: bring a burning splint to the mouth of the tube — the gas burns with a pop sound, $2{\text{H}}_2(g)+{\text{O}}_2(g)\to 2{\text{H}}_2\text{O}(l)$, and the flame is not supported further, because hydrogen burns but does not support combustion.

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9. Q69 Exemplar · Long Answer Questions
   What happens when a piece of

   1. (a)zinc metal is added to copper sulphate ​solution?
   2. (b)​aluminium metal is added to ​dilute hydrochloric ​acid?
   3. (c)​silver metal ​is added to copper ​sulphate solution? Also, write the balanced chemical equation if the reaction occurs
   Show answer

   Zinc displaces copper, aluminium displaces hydrogen from the acid, and silver does not react with copper sulphate.

   (a) Zinc lies above copper in the reactivity series, so it displaces copper from the solution: the blue colour of the copper sulphate slowly fades, a reddish-brown deposit of copper forms on the zinc, and the mixture warms slightly. $\text{Zn}(s)+{\text{CuSO}}_4(aq)\to {\text{ZnSO}}_4(aq)+\text{Cu}(s)$; (b) Aluminium lies well above hydrogen, so it displaces hydrogen from the acid: there is brisk effervescence of hydrogen gas, the metal dissolves and the mixture becomes hot. $2\text{Al}(s)+6\text{HCl}(aq)\to 2{\text{AlCl}}_3(aq)+3{\text{H}}_2(g)$; (c) Silver lies below copper in the reactivity series, so it cannot displace copper. No reaction takes place — the blue colour is unchanged and the silver is unaffected, so no equation can be written.

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10. Q70 Exemplar · Long Answer Questions
    What ​happens when ​zinc granules are treated with dilute ​solution of ${\text{H}}_2{\text{SO}}_4$, $\text{HCl}$, ${\text{HNO}}_3$, $\text{NaCl}$ ​and $\text{NaOH}$, ​also ​write the ​chemical equations if reaction occurs.
    Show answer

    Zinc gives hydrogen with dilute ${\text{H}}_2{\text{SO}}_4$, $\text{HCl}$ and $\text{NaOH}$, an oxide of nitrogen with ${\text{HNO}}_3$, and no reaction with $\text{NaCl}$.

    Dilute ${\text{H}}_2{\text{SO}}_4$: zinc is above hydrogen, so hydrogen is evolved and zinc sulphate is formed — $\text{Zn}(s)+{\text{H}}_2{\text{SO}}_4(aq)\to {\text{ZnSO}}_4(aq)+{\text{H}}_2(g)$. Dilute $\text{HCl}$: the same, giving zinc chloride — $\text{Zn}(s)+2\text{HCl}(aq)\to {\text{ZnCl}}_2(aq)+{\text{H}}_2(g)$. Dilute ${\text{HNO}}_3$: the zinc dissolves, but no hydrogen is evolved, because nitric acid is a strong oxidising agent and oxidises the hydrogen produced to water while its own nitrogen is reduced to an oxide of nitrogen — with very dilute acid, $4\text{Zn}(s)+10{\text{HNO}}_3(aq)\to 4{\text{Zn(NO}}_3{)}_2(aq)+{\text{N}}_2\text{O}(g)+5{\text{H}}_2\text{O}(l)$, and with dilute acid the reduction product is nitric oxide, $3\text{Zn}(s)+8{\text{HNO}}_3(aq)\to 3{\text{Zn(NO}}_3{)}_2(aq)+2\text{NO}(g)+4{\text{H}}_2\text{O}(l)$. Dilute $\text{NaCl}$: no reaction, because sodium chloride is a neutral salt and sodium is far more reactive than zinc, so zinc cannot displace it. Dilute $\text{NaOH}$: hydrogen is evolved, because zinc is amphoteric and reacts with alkalis as well as with acids, dissolving as sodium zincate — $\text{Zn}(s)+2\text{NaOH}(aq)\to {\text{Na}}_2{\text{ZnO}}_2(aq)+{\text{H}}_2(g)$.

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11. Q71 Exemplar · Long Answer Questions
    On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, white precipitate is obtained.

    1. (a)​Write a balanced ​chemical equation of the reaction involved
    2. (b)What other name can ​be given to ​this precipitation reaction?
    3. (c)On ​adding dilute hydrochloric acid ​to the reaction ​mixture, white ​precipitate disappears. Why?
    Show answer

    Barium sulphite precipitates in a double displacement reaction; hydrochloric acid dissolves it, releasing ${\text{SO}}_2$ gas.

    (a) ${\text{BaCl}}_2(aq)+{\text{Na}}_2{\text{SO}}_3(aq)\to {\text{BaSO}}_3(s)+2\text{NaCl}(aq)$ — the white precipitate is barium sulphite, ${\text{BaSO}}_3$; (b) It can also be called a double displacement reaction, since the two compounds simply exchange their ions; (c) Sulphite is the anion of the weak acid ${\text{H}}_2{\text{SO}}_3$, so the much stronger hydrochloric acid takes it out of the solid, forming soluble barium chloride and sulphurous acid. The sulphurous acid at once breaks up into water and sulphur dioxide, which escapes as a gas with the smell of burning sulphur, so the change cannot reverse and the white precipitate dissolves completely: ${\text{BaSO}}_3(s)+2\text{HCl}(aq)\to {\text{BaCl}}_2(aq)+{\text{H}}_2\text{O}(l)+{\text{SO}}_2(g)$.

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12. Q72 Exemplar · Long Answer Questions
    You are ​provided with two containers made ​up of copper ​and aluminium. You are also provided ​with solutions of dilute $\text{HCl}$, dilute ${\text{HNO}}_3$, ${\text{ZnCl}}_2$ and ${\text{H}}_2\text{O}$. In ​which of the ​above containers ​these solutions can be kept?
    Show answer

    The copper container can hold dilute $\text{HCl}$, ${\text{ZnCl}}_2$ and water but not dilute ${\text{HNO}}_3$; the aluminium container can hold only water.

    In the copper container: dilute $\text{HCl}$, ${\text{ZnCl}}_2$ solution and ${\text{H}}_2\text{O}$ can be kept. Copper lies below hydrogen, so dilute hydrochloric acid does not attack it; copper lies below zinc, so it cannot displace zinc from zinc chloride; and copper does not react with water. Dilute ${\text{HNO}}_3$ cannot be kept in it, because nitric acid is an oxidising acid and dissolves copper even though copper is below hydrogen: $3\text{Cu}(s)+8{\text{HNO}}_3(aq)\to 3{\text{Cu(NO}}_3{)}_2(aq)+2\text{NO}(g)+4{\text{H}}_2\text{O}(l)$. In the aluminium container: only ${\text{H}}_2\text{O}$ can be kept, since aluminium is protected from cold water by its oxide film. Dilute $\text{HCl}$ attacks it because aluminium lies above hydrogen, $2\text{Al}(s)+6\text{HCl}(aq)\to 2{\text{AlCl}}_3(aq)+3{\text{H}}_2(g)$; dilute ${\text{HNO}}_3$ attacks it as well; and ${\text{ZnCl}}_2$ solution attacks it because aluminium lies above zinc and displaces it, $2\text{Al}(s)+3{\text{ZnCl}}_2(aq)\to 2{\text{AlCl}}_3(aq)+3\text{Zn}(s)$.

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13. Q73 CBSE 2026
    The volume ​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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14. Q74 CBSE 2026
    When ​lead ​nitrate is heated ​in a ​boiling ​tube, we observe ​:

    1. (a)brown fumes of ${\text{O}}_2$ gas
    2. (b)white fumes of ${\text{NO}}_2$ gas
    3. (c)yellow fumes of lead oxide
    4. (d)brown fumes of ${\text{NO}}_2$ gas
    Show answer

    (d) brown fumes of ${\text{NO}}_2$ gas

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15. Q75 CBSE 2026
    1. (i)${\text{AgNO}}_3+\text{NaCl}⟶{\text{NaNO}}_3+\text{AgCl}$
    2. (ii)${\text{K}}_2{\text{SO}}_4+{\text{BaCl}}_2⟶{\text{BaSO}}_4+2\text{KCl}$ Which of ​the ​following options ​clearly ​describes both the ​reactions ​?

    1. (a)(i) is double displacement, (ii) is displacement reaction.
    2. (b)Both, (i) and (ii) are displacement reactions and precipitation reactions.
    3. (c)Both, (i) and (ii) are double displacement reactions and precipitation reactions.
    4. (d)(i) is displacement, (ii) is double displacement reaction.
    Show answer

    (c) Both, (i) and (ii) are double displacement reactions and precipitation reactions.

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16. Q76 CBSE 2026
    Which ​of the ​following ​are combination reactions ​?

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

    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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17. Q77 CBSE 2026
    Which of the following ​is an ​example of a displacement ​reaction ​?

    1. (a)The combustion of natural gas
    2. (b)The reaction of a reactive metal with dilute acid
    3. (c)The electrolysis of water
    4. (d)The reaction of quick lime with water
    Show answer

    (b) The reaction of a reactive metal with dilute acid

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18. Q78 CBSE 2026
    The ​volume ratio ​of ​hydrogen and oxygen gases liberated during electrolysis ​of ​water ​is :

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

    (b) $2:1$

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19. Q79 CBSE 2026
    Which ​of ​the ​following ​is not an ​exothermic ​reaction ​?

    1. (a)Decomposition of vegetable matter into compost
    2. (b)Reaction of water with quick lime
    3. (c)Respiration process taking place in the cells of our body
    4. (d)Decomposition of calcium carbonate to calcium oxide and carbon dioxide
    Show answer

    (d) Decomposition of calcium carbonate to calcium oxide and carbon dioxide

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20. Q80 CBSE 2026
    If an iron ​nail is ​kept in ​an ​aqueous solution ​of copper sulphate ​:

    1. (a)Iron gets precipitated in the solution.
    2. (b)Displacement reaction takes place.
    3. (c)Double-displacement reaction takes place.
    4. (d)No chemical reaction occurs.
    Show answer

    (b) Displacement reaction takes place.

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