Which among the following shows positive deviation from Raoult's law?

The correct answer is Option (1) → 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 in the pure liquids. Molecules escape more easily, so the vapour pressure is higher than Raoult's law predicts; mixing absorbs heat (\( \Delta_{mix}H \gt 0 \)) and the volume increases (\( \Delta_{mix}V \gt 0 \)). In negative deviation the A–B attractions are stronger and everything is reversed.

Ethanol and acetone fit the first case: acetone weakens the hydrogen bonding between ethanol molecules, so the A–B attractions are weaker and the solution shows positive deviation.

Why the other options are incorrect

• Option (2) Bromoethane and Chloroethane — Incorrect. The two haloalkanes are alike in size and polarity, so the A–B attractions are nearly the same as A–A and B–B; the solution is nearly ideal.

• Option (3) n-Hexane and n-Heptane — Incorrect. Two neighbouring alkanes held by similar van der Waals forces form a nearly ideal solution.

• Option (4) Chloroform and Acetone — Incorrect. These show negative deviation: the hydrogen of chloroform forms a hydrogen bond with the oxygen of acetone, \( \ce{O=C(CH3)2} \), so the A–B attraction is stronger and the vapour pressure falls.

NCERT (2026-27 edition):
Non-ideal Solutions – …

Note: The same/similar question has been asked in different years by NTA as per the details given below: