Class 10 Science · Chapter 1

Chemical Reactions and Equations

298 Class 10 Science questions on Chemical Reactions and Equations from the NCERT textbook, NCERT Exemplar and CBSE board papers 2018–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 7 of 15

1. Q121 CBSE 2025
   The main observations while performing the ​experiment of burning magnesium ribbon in air are :

   1. (i)Magnesium ribbon burns with a dazzling white flame.
   2. (ii)A white powder is ​formed.
   3. (iii)​Magnesium ribbon ​vapourises.
   4. (iv)​Aqueous solution of ​the white powder turns blue litmus to ​red.

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

   (c) (i) and (ii)

   Step-by-step solution →
2. Q122 CBSE 2025
   In the following experimental setup of electrolysis of water, if P and Q are the gases collected in the ​test tubes enclosing the electrodes R and S, then select ​the ​option/options in ​which the matching is correct :

   1. (i)​P - ​Oxygen gas ​, R ​- Anode
   2. (ii)P - Hydrogen gas , R - Cathode
   3. (iii)Q - Hydrogen gas , S - Cathode
   4. (iv)Q - Oxygen gas , S - Anode

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

   (c) (i) and (iii)

   Step-by-step solution →
3. Q123 CBSE 2025
   A student has ​dipped some ​iron nails ​in an aqueous solution of copper sulphate. After about ​25 ​minutes, he ​would observe that the colour ​of the solution has changed from.

   1. (a)blue to colourless
   2. (b)blue to pale green
   3. (c)pale green to blue
   4. (d)pale green to colourless
   Show answer

   (b) blue to pale green

   Step-by-step solution →
4. Q124 CBSE 2025
   In ​the ​electrolysis ​of ​water, ​the mass ratio of hydrogen and ​oxygen gases liberated at the electrodes is ​:

   1. (a)$1:2$
   2. (b)$1:4$
   3. (c)$1:8$
   4. (d)$1:16$
   Show answer

   (c) $1:8$

   Step-by-step solution →
5. Q125 CBSE 2025
   Example ​of ​thermal ​decomposition reaction ​are

   1. (i)$2\text{AgCl}\to 2\text{Ag}+{\text{Cl}}_2$
   2. (ii)${\text{CaCO}}_3\to \text{CaO}+{\text{CO}}_2$
   3. (iii)$2{\text{H}}_2\text{O}\to 2{\text{H}}_2+{\text{O}}_2$
   4. (iv)$2{\text{KClO}}_3\to 2\text{KCl}+3{\text{O}}_2$

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

   (d) (ii) and (iv)

   Step-by-step solution →
6. Q126 CBSE 2025
   Select ​from ​the following decomposition ​reactions in which the source ​of energy for ​decomposition is ​heat :

   1. (i)${\text{CaCO}}_3⟶\text{CaO}+{\text{CO}}_2$
   2. (ii)$2{\text{H}}_2\text{O}⟶2{\text{H}}_2+{\text{O}}_2$
   3. (iii)$2\text{AgBr}⟶2\text{Ag}+{\text{Br}}_2$
   4. (iv)$2{\text{FeSO}}_4⟶{\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$

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

   (d) (i) and (iv)

   Step-by-step solution →
7. Q127 CBSE 2025
   In ​which one of ​the following ​decomposition reactions energy ​is released ​?

   1. (a)Electrolysis of water
   2. (b)Decomposition of silver chloride
   3. (c)Decomposition of vegetable matter into compost
   4. (d)Decomposition of lead nitrate
   Show answer

   (c) Decomposition of vegetable matter into compost

   Step-by-step solution →
8. Q128 CBSE 2025
   Select ​exothermic processes from the following ​:

   1. (i)Dilution of ​acid
   2. (ii)Burning ​of ​natural gas
   3. (iii)Evaporation ​of water
   4. (iv)Electrolysis ​of water

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

   (a) (i) and (ii)

   Step-by-step solution →
9. Q129 CBSE 2025
   Select from ​the ​following ​a ​statement ​which ​is not ​true about burning of magnesium ribbon ​in air :

   1. (a)It burns with a dazzling white flame.
   2. (b)A white powder is formed on burning.
   3. (c)It is an endothermic reaction.
   4. (d)It is an example of a combination reaction.
   Show answer

   (c) It is an endothermic reaction.

   Step-by-step solution →
10. Q130 CBSE 2025
    The values of $a$, $b$, $c$ ​and $d$ in ​the ​following balanced ​chemical equation are respectively ​: $$a{\text{Pb(NO}}_3{\text{)}}_2\stackrel{\text{heat}}{\to }b\text{PbO}+c{\text{NO}}_2+d{\text{O}}_2$$

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

    (d) 2, 2, 4, 1

    Step-by-step solution →
11. Q131 CBSE 2025
    The colour of the solution observed after about 1 hour ​of ​placing ​iron nails ​in copper ​sulphate solution ​is

    1. (a)Blue
    2. (b)Pale green
    3. (c)Yellow
    4. (d)Reddish brown
    Show answer

    (b) Pale green

    Step-by-step solution →
12. Q132 CBSE 2025
    Two statements are given — one ​labelled as Assertion ​(A) ​and the other labelled ​as ​Reason (R). Select the correct answer ​to this question from the codes (a), (b), (c) ​and (d) as given below.

    Assertion (A): Decomposition reactions are generally endothermic reactions.

    Reason (R): Decomposition of organic matter into compost is an exothermic process.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer

    (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

    Step-by-step solution →
13. Q133 CBSE 2025
    Two statements are given ​below — one ​labelled as Assertion ​(A) and the other labelled ​as Reason (R). Select the ​correct ​answer to this question from the ​codes (a), (b), ​(c) and (d) as given below.

    Assertion (A): Decomposition of vegetable matter into compost is an exothermic reaction.

    Reason (R): Decomposition reactions need energy to break down the reactants.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer

    (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

    Step-by-step solution →
14. Q134 CBSE 2025
    Question Nos. ​17 ​to ​20 ​consists of two ​statements – Assertion ​(A) and Reason (R). Answer these questions ​selecting the ​appropriate option from (a), (b), (c) and (d) given below :

    Assertion (A): Silver chloride turns grey in sunlight.

    Reason (R): Decomposition of silver chloride into silver and chlorine takes place by sunlight.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer

    (a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

    Step-by-step solution →
15. Q135 CBSE 2025
    This question consists ​of ​two ​statements – Assertion ​(A) and Reason ​(R). Answer this question selecting the ​appropriate option ​(a), (b), ​(c) and (d) as given below :

    Assertion (A): All exothermic reactions are accompanied with evolution of heat and light.

    Reason (R): Combination reactions may or may not be exothermic.

    1. (a)Both, Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
    2. (b)Both, Assertion (A) and Reason (R) are true, and Reason (R) is not the correct explanation of Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer

    (d) Assertion (A) is false, but Reason (R) is true.

    Step-by-step solution →
16. Q136 CBSE 2025
    A ​student performs ​the following experiment ​in his ​school laboratory. List two observations ​to justify that in this ​experiment ​a chemical change has taken ​place.

    Show answer

    A gas (hydrogen) bubbles off and the flask turns warm, showing that a chemical change has taken place.

    Observation 1: A colourless, odourless gas is given off. Bubbles rise steadily from the surface of the zinc granules and escape through the glass tube, and the gas burns with a pop when a lighted splint is brought to the mouth of the tube, showing it is hydrogen. Observation 2: The flask becomes warm to the touch, so heat is being given out and the reaction is exothermic. Both show that new substances have been formed: $\text{Zn}+{\text{H}}_2{\text{SO}}_4\to {\text{ZnSO}}_4+{\text{H}}_2$. (The zinc granules also gradually shrink and dissolve away, which is an acceptable third observation.)

    Step-by-step solution →
17. Q137 CBSE 2025
    When magnesium ribbon is ​burnt in oxygen, ​an ash of white colour ​is ​produced. Name the type of reaction ​giving justification for your answer. Write the balanced chemical equation for the reaction that occurs.
    Show answer

    It is a combination reaction: magnesium and oxygen combine to form a single product, magnesium oxide, the white ash.

    It is a combination reaction, because two substances — magnesium and oxygen — combine to form a single product, magnesium oxide, the white ash. The balanced equation is $2\text{Mg}(s)+{\text{O}}_2(g)\stackrel{\text{heat}}{\to }2\text{MgO}(s)$. The same reaction is also an oxidation of magnesium, and it gives out heat and light, so it is exothermic.

    Step-by-step solution →
18. Q138 CBSE 2025
    Translate ​the following ​statements into balanced chemical equations :

    1. (a)Aluminium ​reacts with copper chloride to form aluminium chloride and ​copper.
    2. (b)Zinc reacts ​with sodium hydroxide ​to ​give sodium zincate and hydrogen gas.
    Show answer

    The balanced equations are $2\text{Al}+3{\text{CuCl}}_2\to 2{\text{AlCl}}_3+3\text{Cu}$ and $\text{Zn}+2\text{NaOH}\to {\text{Na}}_2{\text{ZnO}}_2+{\text{H}}_2$.

    (a) $2\text{Al}(s)+3{\text{CuCl}}_2(aq)\to 2{\text{AlCl}}_3(aq)+3\text{Cu}(s)$; (b) $\text{Zn}(s)+2\text{NaOH}(aq)\to {\text{Na}}_2{\text{ZnO}}_2(aq)+{\text{H}}_2(g)$

    Step-by-step solution →
19. Q139 CBSE 2025
    Define oxidation. ​Identify ​and name ​the ​substance oxidised in ​the following reaction : $$\text{CuO}+{\text{H}}_2\to \text{Cu}+{\text{H}}_2\text{O}$$
    Show answer

    Oxidation is gain of oxygen or loss of hydrogen; in the reaction hydrogen, ${\text{H}}_2$, is oxidised to water.

    Oxidation is the gain of oxygen by a substance, or the loss of hydrogen from it, during a chemical reaction. In $\text{CuO}+{\text{H}}_2\to \text{Cu}+{\text{H}}_2\text{O}$ the substance oxidised is hydrogen, ${\text{H}}_2$: it gains oxygen and is oxidised to water. Copper oxide loses its oxygen and is reduced to copper.

    Step-by-step solution →
20. Q140 CBSE 2025
    A crystalline substance of ​green colour 'X' emits gases of characteristic ​odour when heated over a flame. It first ​loses water and changes colour. ​On further heating, it decomposes and ​produces a solid compound Y.

    1. (a)Identify 'X' and 'Y'.
    2. (b)State the ​change in colour observed ​when 'X' is heated.
    Show answer

    X is ferrous sulphate crystals and Y is ferric oxide; the colour changes from green to white and then reddish brown.

    (a) $X$ is ferrous sulphate crystals, ${\text{FeSO}}_4\cdot 7{\text{H}}_2\text{O}$ (green vitriol), and $Y$ is ferric oxide, ${\text{Fe}}_2{\text{O}}_3$. (b) The green crystals first turn white (dirty white) as the water of crystallisation is driven off, and on further heating the residue turns reddish brown.

    Step-by-step solution →