Skip to content

(i) Define a decomposition… (Class 10 Science 2024, Q34b)

questionbuddy.in/q/cbse-10-science-2024-31-4-1-q34b-define-decomposition-reaction-how-can

ScienceCBSE Class 10 · Board exam 2024, set 31/4/1 · 5 marks · Long answer

CBSE Class 10 Science 2024: question 34(b)

Set in the CBSE Class 10 board exam of 2024, paper 31/4/1, section D, question 34(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q34(b) · 5 marks · Long answerChemical Reactions and Equations

(i) Define a decomposition reaction. How can we say that (I) electrolysis of water, and (II) blackening of silver bromide when exposed to sunlight, are decomposition reactions ? Mention the type of energy involved in each case.
(ii) "The type of reactions in which (I) calcium oxide is formed, and (II) calcium hydroxide is formed are opposite reactions to each other." Justify this statement with the help of chemical equations.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 31/4/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

In each case, count how many reactants and how many products there are.

Hint 2

CaO is made by heating limestone, and Ca(OH)₂ by adding water to CaO. Compare the heat change and the reaction type.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i) A decomposition reaction is one in which a single reactant breaks down into two or more simpler products.

    1 mark

  2. (I) In electrolysis, water (a single reactant) splits into hydrogen and oxygen: . So it is a decomposition reaction.

    ½ mark

  3. Energy involved: electrical energy.

    ½ mark

  4. (II) Silver bromide breaks down into silver (which is black, hence the blackening) and bromine: . So it is a decomposition reaction.

    ½ mark

  5. Energy involved: light energy.

    ½ mark

  6. (ii)(I) Formation of calcium oxide:

    ½ mark

  7. This is a decomposition reaction, and it is endothermic (heat is absorbed).

    ½ mark

  8. (II) Formation of calcium hydroxide: + heat

    ½ mark

  9. This is a combination reaction, and it is exothermic (heat is released). Decomposition is the opposite of combination, and endothermic is the opposite of exothermic, so the two reactions are opposite to each other.

    ½ mark

Answer: (i) Decomposition: a single reactant breaks into simpler products. (I) H₂O → H₂ + O₂ using electrical energy; (II) AgBr → Ag + Br₂ using light energy. (ii) CaCO₃ → CaO + CO₂ is decomposition/endothermic; CaO + H₂O → Ca(OH)₂ + heat is combination/exothermic, so they are opposite.

Where the marks are

  • Definition of decomposition (1 mark)
  • Water splits into H₂ and O₂ (½ mark); electrical energy (½ mark)
  • AgBr splits into Ag and Br₂ (½ mark); light energy (½ mark)
  • Equation for formation of CaO (½ mark); identified as decomposition/endothermic (½ mark)
  • Equation for formation of Ca(OH)₂ (½ mark); identified as combination/exothermic (½ mark)

Revise the chapter

Practise this chapterBoard-style questions from Chemical Reactions and Equations, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 31/4/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.