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B. I. Refer to the… (Class 10 Science SQP 2026-27, Q29B)

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ScienceCBSE Class 10 · Sample paper, session 2026-27 · 5 marks · Long answer

CBSE Class 10 Science sample paper 2026-27: question 29(B)

From the official CBSE sample question paper for 2026-27, section B, question 29(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q29(B) · 5 marks · Long answerCompetency-basedChemical Reactions and Equations

B. I. Refer to the dialogue between Daljeet and Avik in the chemistry lab. They have learnt about decomposition reactions. Who has understood decomposition reactions correctly? Justify your answer. Daljeet: During a decomposition reaction, a substance decomposes to give two simpler compounds. decomposes on heating to magnesium oxide and carbon dioxide. Avik: No, during a decomposition reaction, a substance decomposes to give two elements. Water on electrolysis gives hydrogen and oxygen, both are elements. II. Analyse the observations given in following two cases. Are these observations correct? Give reasons to support your answer. (i) Iron fillings are added to an aqueous solution of magnesium sulphate. Observation: The colourless solution turns greenish due to formation of iron sulphate. (ii) Aluminium metal is added to dilute sulphuric acid. Observation: Brisk effervescence are seen due to evolution of a gas.

  1. I

    Refer to the dialogue between Daljeet and Avik. Who has understood decomposition reactions correctly? Justify your answer.

    0 marks
  2. II

    Analyse the observations given in following two cases. (i) Iron fillings are added to an aqueous solution of magnesium sulphate. Observation: The colourless solution turns greenish due to formation of iron sulphate. (ii) Aluminium metal is added to dilute sulphuric acid. Observation: Brisk effervescence are seen due to evolution of a gas.

    0 marks

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Look at the products in each student's example: one gives compounds and the other gives elements.

Hint 2

Use the reactivity series to check whether iron can displace magnesium.

Hint 3

Name the gas a metal gives with a dilute acid, and recall which gas is linked with brisk effervescence.

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. Neither has understood it completely; each is only partially correct. In a decomposition reaction, a single compound breaks down into two or more simpler substances. These products may be elements or compounds.

    1 mark

  2. Daljeet's example is valid (), but products need not always be compounds.

    Avik's example is also valid (). However, products need not always be elements, as Daljeet's example shows. So each one's rule is too narrow.

    2 marks

  3. II (i). The observation is incorrect. Iron is less reactive than magnesium, so it cannot displace Mg from . No reaction occurs, and the solution stays colourless.

    1 mark

  4. II (ii). The observation is incorrect. The gas evolved is hydrogen:

    Only a few bubbles are seen, because the reaction is slow at first owing to the protective oxide layer on aluminium. Brisk effervescence is typical of released from carbonates.

    1 mark

Answer: I. Both are only partially correct: in decomposition, a single compound breaks into simpler substances that may be elements or compounds. II. (i) Incorrect, as Fe is less reactive than Mg, so no reaction occurs; (ii) Incorrect, as is evolved with only a few bubbles, not brisk effervescence.

Where the marks are

  • States that both are partial and gives the correct definition (single compound giving simpler substances, either elements or compounds) (2 marks)
  • Uses both examples, the and water electrolysis cases, as decomposition (1 mark)
  • (i) Incorrect, because Fe is less reactive than Mg and cannot displace it (1 mark)
  • (ii) Incorrect, because gives only a few bubbles; brisk effervescence goes with (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 paperSample paper 2026-27: 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.