Chemical Reactions & Equations
1. Electrolysis of water is a decomposition reaction. The mass ratio (MH : MO ) of
hydrogen and oxygen gases liberated at the electrodes during electrolysis of water
is: (1 Mark) (2025)
(A) 8 : 1
(B) 2 : 1
(C) 1 : 2
(D) 1 : 8
Two statements are given — one labelled as Assertion (A) and the other
labelled as Reason (R). Select the correct answer to these questions from the
codes (A), (B), (C) and (D) as given below.
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct
explanation of Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct
explanation of Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.
2. Assertion (A) : Decomposition reactions are generally endothermic reactions.
Reason (R) : Decomposition of organic matter into compost is an exothermic
process. (1 Mark) (2025)
3. 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. (2 Mark) (2025)
, Answers
1. D. /1: 8
2. (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct
explanation of Assertion (A).
3. • Evolution of gas
• Change / Rise in temperature
,1. When 2 mL of sodium hydroxide solution is added to few pieces of
granulated zinc in a test tube and then warmed, the reaction that occurs can
be written in the form of a balanced chemical equation as : (2024)
Answer. (b) / 2 𝑁𝑎𝑂𝐻 + 𝑍𝑛 → 𝑁𝑎2𝑍𝑛𝑂2 + 𝐻2
2. Select from the following a decomposition reaction in which source of
energy for decomposition is light: (2024)
Answer. (c) / ⎯⎯→ 2 2 AgBr 2 Ag + Br2
3.
(2024)
The reaction given above is a redox reaction because in this case :
(a) MnO2 is oxidised and HCl is reduced.
(b) HCl is oxidised.
(c) MnO2 is reduced.
(d) MnO2 is reduced and HCl is oxidised.
Answer. (d) / MnO2 is reduced and HCl is oxidised
, 4. Assertion (A): Hydrogen gas is not evolved when zinc reacts with nitric acid.
Reason (R): Nitric acid oxidises the hydrogen gas produced to water and itself
gets reduced. (2024)
Answer. (a) /Both Assertion (A) and Reason (R) are true and Reason (R) is the
correct explanation of Assertion (A).
5. Name the type of chemical reaction in which calcium oxide reacts with
water. Justify your answer by giving balanced chemical equation for the
chemical reaction. (2024)
Answer. Combination reaction – Single product is formed (or any other)
6. Write one chemical equation each for the chemical reaction in which the
following have taken place : (2024)
(i) Change in colour
(ii) Change in temperature
(iii) Formation of precipitate
Mention colour change/temperature change (rise/fall)/compound
precipitated along with equation.
Answer. (i) Change in colour: The solution will become green in colour.
(or any other reaction which shows change in colour)
(ii) Change in temperature: The temperature will increase.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) + Heat
(or any other reaction which shows change in temperature)
(iii) Formation of precipitate: Yellow precipitate of PbI2 is formed.
Pb(NO3)2 (aq) + 2 KI(aq) → PbI2(s) + 2 KNO3(aq) Yellow
(or any other reaction which shows formation of precipitate)
1.1 Chemical Equations
1. Electrolysis of water is a decomposition reaction. The mass ratio (MH : MO ) of
hydrogen and oxygen gases liberated at the electrodes during electrolysis of water
is: (1 Mark) (2025)
(A) 8 : 1
(B) 2 : 1
(C) 1 : 2
(D) 1 : 8
Two statements are given — one labelled as Assertion (A) and the other
labelled as Reason (R). Select the correct answer to these questions from the
codes (A), (B), (C) and (D) as given below.
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct
explanation of Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct
explanation of Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.
2. Assertion (A) : Decomposition reactions are generally endothermic reactions.
Reason (R) : Decomposition of organic matter into compost is an exothermic
process. (1 Mark) (2025)
3. 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. (2 Mark) (2025)
, Answers
1. D. /1: 8
2. (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct
explanation of Assertion (A).
3. • Evolution of gas
• Change / Rise in temperature
,1. When 2 mL of sodium hydroxide solution is added to few pieces of
granulated zinc in a test tube and then warmed, the reaction that occurs can
be written in the form of a balanced chemical equation as : (2024)
Answer. (b) / 2 𝑁𝑎𝑂𝐻 + 𝑍𝑛 → 𝑁𝑎2𝑍𝑛𝑂2 + 𝐻2
2. Select from the following a decomposition reaction in which source of
energy for decomposition is light: (2024)
Answer. (c) / ⎯⎯→ 2 2 AgBr 2 Ag + Br2
3.
(2024)
The reaction given above is a redox reaction because in this case :
(a) MnO2 is oxidised and HCl is reduced.
(b) HCl is oxidised.
(c) MnO2 is reduced.
(d) MnO2 is reduced and HCl is oxidised.
Answer. (d) / MnO2 is reduced and HCl is oxidised
, 4. Assertion (A): Hydrogen gas is not evolved when zinc reacts with nitric acid.
Reason (R): Nitric acid oxidises the hydrogen gas produced to water and itself
gets reduced. (2024)
Answer. (a) /Both Assertion (A) and Reason (R) are true and Reason (R) is the
correct explanation of Assertion (A).
5. Name the type of chemical reaction in which calcium oxide reacts with
water. Justify your answer by giving balanced chemical equation for the
chemical reaction. (2024)
Answer. Combination reaction – Single product is formed (or any other)
6. Write one chemical equation each for the chemical reaction in which the
following have taken place : (2024)
(i) Change in colour
(ii) Change in temperature
(iii) Formation of precipitate
Mention colour change/temperature change (rise/fall)/compound
precipitated along with equation.
Answer. (i) Change in colour: The solution will become green in colour.
(or any other reaction which shows change in colour)
(ii) Change in temperature: The temperature will increase.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) + Heat
(or any other reaction which shows change in temperature)
(iii) Formation of precipitate: Yellow precipitate of PbI2 is formed.
Pb(NO3)2 (aq) + 2 KI(aq) → PbI2(s) + 2 KNO3(aq) Yellow
(or any other reaction which shows formation of precipitate)
1.1 Chemical Equations
