Predict which of the following mixtures will show a positive deviation from Raoult's Law?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Ethanol and acetone
A solution shows positive deviation from Raoult’s law when the A–B attractions are weaker than the A–A and B–B attractions. The molecules escape more easily, so the vapour pressure is higher than Raoult’s law predicts.
In ethanol and acetone, acetone breaks some of the hydrogen bonds of ethanol, so the A–B attractions are weaker than the A–A attractions.
Why the other options are incorrect
Option (1) Nitric acid and water — Incorrect. Strong attractions (hydrogen bonding and ionisation) form between \( \ce{HNO3} \) and water, so the mixture shows negative deviation.
Option (3) Phenol and aniline — Incorrect. The –OH of phenol forms a hydrogen bond with the nitrogen of aniline, which is stronger than the attractions in the pure liquids: negative deviation.
Option (4) Chloroform and acetone — Incorrect. The hydrogen of chloroform forms a hydrogen bond with the oxygen of acetone, so the vapour pressure falls: negative deviation.
NCERT (2026-27 edition):
Non-ideal Solutions – …
