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What is meant by positive… (Class 12 Chemistry 2025, Q17)

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ChemistryCBSE Class 12 · Board exam 2025, set 56/7/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2025: question 17

Set in the CBSE Class 12 board exam of 2025, paper 56/7/1, 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 is meant by positive deviation from Raoult’s law ? Give an example. What type of azeotrope is formed by positive deviation ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 56/7/1.

Hints

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

Hint 1

Compare the actual vapour pressure with the value Raoult's law predicts.

Hint 2

Think about whether the A–B attractions are weaker or stronger than A–A and B–B.

Hint 3

If vapour pressure is higher, is the boiling point of the azeotrope lower or higher?

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. Positive deviation: a solution shows positive deviation from Raoult's law when its vapour pressure is higher than the value predicted by Raoult's law. This happens because the A–B (solute–solvent) attractions are weaker than the A–A and B–B attractions, so molecules escape more easily. For such solutions and .

    1 mark

  2. Example: a mixture of ethanol and acetone. Carbon disulphide + acetone is another example.

    ½ mark

  3. Solutions showing a large positive deviation form a minimum boiling azeotrope. An example is about 95% ethanol–water.

    ½ mark

Answer: Vapour pressure is higher than predicted by Raoult's law; e.g. ethanol + acetone; minimum boiling azeotrope.

Where the marks are

  • States that vapour pressure of the solution is higher than expected for ideal behaviour (1 mark)
  • Gives a correct example such as ethanol + acetone (½ mark)
  • Names minimum boiling azeotrope (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 paper2025 board paper, set 56/7/1: 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.