For non-ideal solutions exhibiting positive deviation :
A. The vapour pressure is lower than that predicted by Raoult's law.
B. The vapour pressure is higher than that predicted by Raoult's law.
C. The interactions at the molecular level are weaker than in the pure state.
D. The interactions at the molecular level are stronger than in the pure state.
E. Mixture of chloroform and acetone is a suitable example.
Choose the correct answers from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → B, C only
In positive deviation from Raoult’s law the A–B interactions are weaker than the A–A and B–B interactions, so the molecules escape more easily and the vapour pressure is higher than predicted.
(A) The vapour pressure is lower than that predicted by Raoult’s law — False. A lower vapour pressure is negative deviation.
(B) The vapour pressure is higher than that predicted by Raoult’s law — True. This is what positive deviation means.
(C) The interactions at the molecular level are weaker than in the pure state — True. Weaker A–B attractions raise the escaping tendency.
(D) The interactions at the molecular level are stronger than in the pure state. — False. Stronger A–B attractions cause negative deviation.
(E) Mixture of chloroform and acetone is a suitable example. — False. Hydrogen bonding between \( \ce{CHCl3} \) and acetone makes this a solution with negative deviation (ethanol + acetone is a positive-deviation example).
NCERT (2026-27 edition):
Non-ideal Solutions – …
“… In case of negative deviations from Raoult’s law, the intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure resulting in negative deviations. An example of this type is a mixture of phenol and aniline. In this case the intermolecular hydrogen bonding between phenolic proton and lone pair on nitrogen atom of aniline is stronger than the respective intermolecular hydrogen bonding between similar molecules. Similarly, a mixture of chloroform and acetone forms a solution with negative deviation from Raoult’s law.”