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What type of deviation from… (Class 12 Chemistry 2026, Q17)

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ChemistryCBSE Class 12 · Board exam 2026, set 56/1/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2026: question 17

Set in the CBSE Class 12 board exam of 2026, paper 56/1/1, section B, question 17, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q17 · 2 marks · Very short answerSolutions

What type of deviation from Raoult's law is shown by a mixture of phenol and aniline ? Give reason.
What will happen to the boiling point of the solution on mixing phenol and aniline ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 56/1/1.

Hints

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

Hint 1

Think about which molecule has an acidic H and which has a lone pair.

Hint 2

If the A–B attractions are stronger than A–A and B–B, what happens to the vapour pressure?

Hint 3

Lower vapour pressure means a higher temperature is needed to boil.

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. A phenol–aniline mixture shows negative deviation from Raoult's law.

    ½ mark

  2. Reason: the acidic phenolic –OH hydrogen forms an intermolecular hydrogen bond with the lone pair on the nitrogen of aniline (). These phenol–aniline (A–B) attractions are stronger than the phenol–phenol (A–A) and aniline–aniline (B–B) attractions, so the escaping tendency of molecules falls and the vapour pressure is lower than Raoult's law predicts.

    1 mark

  3. Because the vapour pressure is lowered, the boiling point of the solution increases. It is higher than expected, and such mixtures can form a maximum-boiling azeotrope.

    ½ mark

Answer: Negative deviation, because phenol–aniline H-bonding is stronger than the like–like attractions; the boiling point increases.

Where the marks are

  • Identifies negative deviation (½ mark)
  • Reason: solute–solvent (phenol–aniline) H-bonding stronger than solute–solute and solvent–solvent attractions (1 mark)
  • Boiling point of the solution increases (½ mark)

Revise the chapter

  • Chapter: Solutions (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Solutions, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 56/1/1: 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.