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Why are decomposition… (Class 10 Science 2025, Q27b)

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ScienceCBSE Class 10 · Board exam 2025, set 31/2/1 · 3 marks · Short answer

CBSE Class 10 Science 2025: question 27(b)

Set in the CBSE Class 10 board exam of 2025, paper 31/2/1, section C, question 27(b), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27(b) · 3 marks · Short answerChemical Reactions and Equations

Why are decomposition reactions called the opposite of combination reactions? Write one chemical equation each for these two types of reactions mentioning the name of the reactant(s) and the product(s) involved in the reactions.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/2/1.

Hints

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

Hint 1

Compare the number of reactants and products in each type.

Hint 2

Think of burning carbon in oxygen, and heating limestone.

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. In a combination reaction, two or more reactants join to form a single product. In a decomposition reaction, a single reactant breaks down into two or more products. One joins substances and the other splits them, so the two are opposite processes.

    1 mark

  2. Combination: . Reactants: carbon and oxygen; product: carbon dioxide.

    1 mark

  3. Decomposition: . Reactant: calcium carbonate; products: calcium oxide and carbon dioxide.

    1 mark

Answer: Combination joins reactants into one product, while decomposition splits one reactant into several products. Examples: C + O₂ → CO₂; CaCO₃ → CaO + CO₂.

Where the marks are

  • Explains why the two reactions are opposite (1 mark)
  • Combination equation with reactants and product named (1 mark)
  • Decomposition equation with reactant and products named (1 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 paper2025 board paper, set 31/2/1: 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.