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Combination and decomposition reactions

High importanceUsually 4 marks a paper · mostly MCQs and short answers · asked in 4 of the last papers

Notes are being written

The board questions below are real and complete — they come from the CBSE papers themselves. The written notes for this topic are still being prepared.

Board questions on this topic

9 from the CBSE papers we hold — the exact wording the board used.

2026MCQ·1 mark·Q17

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

2026MCQ·1 mark·Q18

C_6H_{12}O_6(aq) + 6O_2(g) -> 6CO_2(g) + 6H_2O(l). The above reaction is an example of -

  1. (a) Displacement Reaction
  2. (b) Endothermic Reaction
  3. (c) Exothermic Reaction
  4. (d) Neutralisation Reaction

2026Short answer·3 marks·Q26

Account for the following: (a) White silver chloride turns grey when exposed to sunlight. (b) Lead (II) nitrate on heating releases brown fumes. (c) Bags of oil and fat containing food items are flushed with nitrogen gas.

2025MCQ·1 mark·Q1

Electrolysis of water is a decomposition reaction. The mass ratio (M_H : M_O) of hydrogen and oxygen gases liberated at the electrodes during electrolysis of water is :

  1. (a) 8 : 1
  2. (b) 2 : 1
  3. (c) 1 : 2
  4. (d) 1 : 8

2025Assertion-Reason·1 mark·Q17

Assertion (A): Decomposition reactions are generally endothermic reactions. Reason (R): Decomposition of organic matter into compost is an exothermic process. Select the correct answer:

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

2024MCQ·1 mark·Q2

Select from the following a decomposition reaction in which source of energy for decomposition is light:

  1. (a) 2FeSO_4 -> Fe_2O_3 + SO_2 + SO_3
  2. (b) 2H_2O -> 2H_2 + O_2
  3. (c) 2AgBr -> 2Ag + Br_2
  4. (d) CaCO_3 -> CaO + CO_2

2024Very short answer·2 marks·Q21

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.

2024Short answer·3 marks·Q27

Write one chemical equation each for the chemical reaction in which the following have taken place: (i) Change in colour (ii) Change in temperature (iii) Formation of precipitate. Mention colour change/temperature change (rise/fall)/compound precipitated along with equation.

2023MCQ·1 mark·Q2

The emission of brown fumes in the given experimental set-up is due to

Figure printed with question 2 of the 2023 board paper
  1. (a) thermal decomposition of lead nitrate which produces brown fumes of nitrogen dioxide.
  2. (b) thermal decomposition of lead nitrate which produces brown fumes of lead oxide.
  3. (c) oxidation of lead nitrate forming lead oxide and nitrogen dioxide.
  4. (d) oxidation of lead nitrate forming lead oxide and oxygen. [Set-up shows lead nitrate being heated over a burner, giving off brown fumes.]