A carbon compound 'A' is widely used as a preservative in pickles and has a molecular formula C_2H_4O_2. This compound reacts with ethanol to form a sweet smelling compound 'B'. (i) Identify the compound 'A' and write its structure. (ii) Write the chemical equation for the reaction of 'A' with ethanol to form compound 'B'. State the role of presence of an acid in the reaction. (iii) How can we get compound 'A' back from 'B'? (iv) How can 'A' be obtained from ethanol? (v) Name the gas produced when compound 'A' reacts with washing soda.

ScienceCBSE Class 10 · Board exam 2025, set 31/4/1 · 5 marks · Long answer

CBSE Class 10 Science 2025: question 34(B)

Set in the CBSE Class 10 board exam of 2025, 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 answerCarbon and its Compounds

A carbon compound 'A' is widely used as a preservative in pickles and has a molecular formula $C_{2} H_{4} O_{2}$ . This compound reacts with ethanol to form a sweet smelling compound 'B'.

(i)
Identify the compound 'A' and write its structure.
0 marks
(ii)
Write the chemical equation for the reaction of 'A' with ethanol to form compound 'B'. State the role of presence of an acid in the reaction.
0 marks
(iii)
How can we get compound 'A' back from 'B'?
0 marks
(iv)
How can 'A' be obtained from ethanol?
0 marks
(v)
Name the gas produced when compound 'A' reacts with washing soda.
0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/4/1.

Hints

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

Hint 1

Vinegar is a dilute solution of which acid?

Hint 2

An acid and an alcohol give an ester. Reversing this needs a base or water with acid.

Hint 3

Converting an alcohol into a carboxylic acid requires an oxidising agent. Washing soda is a carbonate.

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.

(i) 'A' is **ethanoic acid (acetic acid), $C H_{3} COOH$ **. A dilute solution of it (vinegar) is used as a pickle preservative.
½ mark
(i) Structure: a $C H_{3}$ group is attached to a carbon that is double-bonded to O and single-bonded to an OH group, i.e. $H_{3} C - C (= O) - O - H$ .
½ mark
(ii) Esterification gives the sweet-smelling ester 'B', ethyl ethanoate:
$C H_{3} COOH + C_{2} H_{5} OH Acid C H_{3} COO C_{2} H_{5} + H_{2} O$
½ mark
(ii) Role of acid: concentrated sulphuric acid acts as a catalyst.
½ mark
(iii) Heat the ester 'B' with dilute NaOH (saponification):
$C H_{3} COO C_{2} H_{5} + NaOH \to C H_{3} COONa + C_{2} H_{5} OH$
Acidifying the sodium ethanoate gives ethanoic acid 'A' back. Alternatively, hydrolyse the ester with water in the presence of an acid.
1 mark
(iv) Oxidise ethanol by warming it with alkaline potassium permanganate or acidified potassium dichromate:
$C H_{3} C H_{2} OH Alk. KMn O_{4} + Heat C H_{3} COOH$
1 mark
(v) **Carbon dioxide ( $C O_{2}$ )** is produced:
$2C H_{3} COOH + N a_{2} C O_{3} \to 2C H_{3} COONa + H_{2} O + C O_{2}$
1 mark

Answer: (i) Ethanoic acid, $C H_{3} COOH$ ; (ii) $C H_{3} COOH + C_{2} H_{5} OH \to C H_{3} COO C_{2} H_{5} + H_{2} O$ , acid acts as catalyst; (iii) hydrolysis/saponification of the ester with NaOH, then acidification; (iv) oxidation with alkaline $KMn O_{4}$ or acidified $K_{2} C r_{2} O_{7}$ ; (v) $C O_{2}$ .

Where the marks are

Identifies ethanoic acid and draws its structure (½ + ½ mark)
Correct esterification equation (½ mark)
States the acid acts as a catalyst (½ mark)
Method to get A back: hydrolysis or saponification of the ester (1 mark)
Oxidation of ethanol with alkaline $KMn O_{4}$ or acidified $K_{2} C r_{2} O_{7}$ (1 mark)
Names carbon dioxide (1 mark)

Revise the chapter

Chapter: Carbon and its Compounds (notes, formulas, where marks are lost and every past question)

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 31/4/1: every question, by section.

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CBSE Class 10 Science 2025: question 34(B)

Hints

Step-by-step solution

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