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Give reasons : (1 + 1 = 2)… (Class 12 Chemistry 2025, Q17A)

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

CBSE Class 12 Chemistry 2025: question 17(A)

Set in the CBSE Class 12 board exam of 2025, paper 56/1/1, section B, question 17(A), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q17(A) · 2 marks · Very short answerCompetency-basedSolutions

Give reasons : (1 + 1 = 2)
(a) Cooking is faster in pressure cooker than in an open pan.
(b) On mixing liquid X and liquid Y, volume of the resulting solution decreases. What type of deviation from Raoult's law is shown by the resulting solution ? What change in temperature would you observe after mixing liquids X and Y ?

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

Hints

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

Hint 1

How does the boiling point of a liquid depend on the external pressure?

Hint 2

If the volume contracts on mixing, are the A–B attractions stronger or weaker than A–A and B–B?

Hint 3

Stronger new attractions release energy; think about the sign of ΔH_mix.

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) A pressure cooker is sealed, so the pressure above the water becomes higher than atmospheric pressure. Water boils when its vapour pressure equals the external pressure, so its boiling point rises above 100 °C. The food cooks at this higher temperature, which is why cooking is faster.

    1 mark

  2. (b) The volume decreases on mixing (). This means the X–Y attractions are stronger than the X–X and Y–Y attractions, so the solution shows negative deviation from Raoult's law.

    ½ mark

  3. For negative deviation, heat is released on mixing (, exothermic). The temperature of the mixture rises.

    ½ mark

Answer: (a) Higher pressure raises the boiling point of water, so food cooks faster; (b) negative deviation; temperature increases

Where the marks are

  • Higher pressure gives a higher boiling point, so cooking is faster (1 mark)
  • Identifies negative deviation (½ mark)
  • States that the temperature 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 paper2025 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.