CBSE Class 12 · 2000 · Delhi; also All India 2000 · SolutionsShort Answer

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Give one example each of miscible liquid pairs showing positive and negative deviations from Raoult's law. Give one reason each for such deviations.

Solution

Positive deviation: ethanol + water (or ethanol + acetone). Reason: the added component breaks hydrogen bonds of the other; the A–B attractions are weaker than A–A and B–B, so molecules escape more easily and the vapour pressure is higher than ideal.
Negative deviation: chloroform + acetone. Reason: a new hydrogen bond forms between the H of CHCl

_{3}

and the O of acetone; the A–B attractions are stronger than A–A and B–B, so escape is harder and the vapour pressure is lower than ideal.

CBSE Class 12 Board · 2000 · Delhi; also All India 2000 · Subjective

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