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

CBSE Class 10 Science 2025: question 34(A)

Set in the CBSE Class 10 board exam of 2025, paper 31/4/1, 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

A carbon compound 'A' on heating with excess conc. $H_{2} S O_{4}$ forms a compound 'B', which on addition of one mole of hydrogen gas in the presence of nickel catalyst forms a compound 'C'. 'C' on combustion in air forms 2 moles of carbon dioxide and 3 moles of water. Identify 'A', 'B' and 'C' and write their structures. Give chemical equations of the reactions involved. Also state the role of concentrated sulphuric acid in the formation of 'B' from 'A'.

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

Work backwards from the combustion products: count the C and H atoms in C.

Hint 2

Adding $H_{2}$ in presence of Ni turns an unsaturated compound into a saturated one.

Hint 3

Heating an alcohol with excess hot conc. $H_{2} S O_{4}$ removes a water molecule.

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.

'C' burns to give 2 mol $C O_{2}$ and 3 mol $H_{2} O$ , so it has 2 C and 6 H atoms. 'C' is **ethane, $C_{2} H_{6}$ **.
Structure: $H_{3} C - C H_{3}$ , i.e. each carbon bonded to three H atoms and to the other carbon by a single bond.
1 mark
'B' adds one mole of $H_{2}$ to give ethane, so 'B' is **ethene, $C_{2} H_{4}$ **.
Structure: $H_{2} C = C H_{2}$ (two carbons joined by a double bond, each carrying two H atoms).
1 mark
'B' is formed from 'A' by dehydration, so 'A' is **ethanol, $C H_{3} C H_{2} OH$ **.
Structure: $H_{3} C - C H_{2} - O - H$ .
1 mark
A → B: $C H_{3} C H_{2} OH Hot conc. H_{2} S O_{4} C H_{2} = C H_{2} + H_{2} O$
½ mark
B → C: $C H_{2} = C H_{2} + H_{2} Ni C H_{3} - C H_{3}$
½ mark
Combustion of C: $C_{2} H_{6} + \frac{7}{2} O_{2} \to 2C O_{2} + 3 H_{2} O$ (or $2 C_{2} H_{6} + 7 O_{2} \to 4C O_{2} + 6 H_{2} O$ )
½ mark
**Role of conc. $H_{2} S O_{4}$ :** it acts as a dehydrating agent and removes a water molecule from ethanol.
½ mark

Answer: A = ethanol ( $C H_{3} C H_{2} OH$ ), B = ethene ( $C H_{2} = C H_{2}$ ), C = ethane ( $C_{2} H_{6}$ ); conc. $H_{2} S O_{4}$ acts as a dehydrating agent.

Where the marks are

A identified as ethanol with its structure (½ + ½ mark)
B identified as ethene with its structure (½ + ½ mark)
C identified as ethane with its structure (½ + ½ mark)
Equation for the dehydration of ethanol (½ mark)
Equation for the hydrogenation of ethene (½ mark)
Equation for the combustion of ethane (½ mark)
Role of conc. $H_{2} S O_{4}$ as a dehydrating agent (½ 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(A)

Hints

Step-by-step solution

Revise the chapter

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