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Answer the following. (Science 2025 31/2/1 Q34a)

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ScienceCBSE Class 10 · Board exam 2025, set 31/2/1 · 5 marks · Long answer

CBSE Class 10 Science 2025: question 34(a)

Set in the CBSE Class 10 board exam of 2025, paper 31/2/1, section D, question 34(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q34(a) · 5 marks · Long answerCarbon and its Compounds

Answer the following.

  1. (i)

    A compound 'X' having two carbon atoms in its molecule turns blue litmus red and 5–8% solution of 'X' in water is widely used as a preservative. Identify the compound 'X' and write its structure.

    1½ marks
  2. (ii)

    Compare its pH nature with a mineral acid.

    ½ mark
  3. (iii)

    'X' on reacting with alcohols produces sweet smelling compounds, used in making perfumes. Name the reaction and write its chemical equation.

    1½ marks
  4. (iv)

    When sodium carbonate is added to 'X', a colourless gas is produced which turns lime water milky. Write the chemical equation for the reaction giving the name of the salt produced.

    1½ marks

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

Which two-carbon acid, as a dilute solution, is used as vinegar for pickles?

Hint 2

Weak acids ionise less. Acid + alcohol gives an ester, and acid + carbonate gives a salt, water and CO₂.

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) X has two carbon atoms and is acidic (turns blue litmus red). A 5–8% solution of it in water is vinegar, which is used as a preservative. So X is ethanoic acid (acetic acid).

    ½ mark

  2. Structure: . In expanded form, one carbon carries three H atoms and is bonded to a second carbon. The second carbon has a double bond to O and a single bond to an –OH group (H–C(H)(H)–C(=O)–O–H).

    1 mark

  3. (ii) Ethanoic acid is a weak acid and ionises only partially. Its pH is therefore higher than that of a mineral acid such as HCl of similar concentration.

    ½ mark

  4. (iii) Name of reaction: esterification.

    ½ mark


  5. (ethanoic acid + ethanol → ethyl ethanoate (ester, sweet smelling) + water)

    1 mark

  6. (iv) . The CO₂ gas turns lime water milky.

    1 mark

  7. Salt produced: sodium ethanoate (sodium acetate), .

    ½ mark

Answer: (i) X = ethanoic acid, CH₃COOH; (ii) pH higher than a mineral acid (weaker acid); (iii) esterification: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O; (iv) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂, salt = sodium ethanoate

Where the marks are

  • Identifies X as ethanoic/acetic acid (½ mark)
  • Correct structure of CH₃COOH (1 mark)
  • pH of X higher than mineral acid (½ mark)
  • Names esterification (½ mark)
  • Correct esterification equation (1 mark)
  • Equation with sodium carbonate (1 mark)
  • Names sodium ethanoate/acetate (½ mark)

Revise the chapter

Practise this chapterBoard-style questions from Carbon and its Compounds, 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.