An ideal solution is one in which the attraction between components of the solution is the same as the interaction between the molecules of each component. Heat is neither absorbed nor evolved during the formation of an ideal solution and the volume of the solution is equal to the sum of the volumes of the component liquids. Raoult’s law states that the partial vapour pressure of each component (say liquid A) of the solution is directly proportional to its mole fraction in the solution. The total vapour pressure over the solution is equal to the sum of the partial vapour pressures of the individual components.
A non-ideal solution is that solution (i) which does not obey Raoult’s law (ii) for which \(\Delta V_{mix}\) is not zero and (iii) for which \(\Delta H_{mix}\) is not zero. In non-ideal solutions, the solute–solvent interactions are weaker or stronger than the solute–solute and solvent–solvent interactions. The non-ideal solutions in which solute–solvent interactions are weaker than the solute–solute and solvent–solvent interactions show positive deviation from Raoult’s law, and those in which they are stronger show negative deviation.
Considering the formation, breaking and strength of hydrogen bonds, which of the following mixtures will show a positive deviation from Raoult’s law?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Methanol 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; molecules then escape more easily and the vapour pressure is higher than predicted.
Pure methanol, like ethanol, is hydrogen bonded. Acetone molecules get in between the methanol molecules and break some of these hydrogen bonds. The attractions in the mixture are weaker, so it shows positive deviation.
Why the other options are incorrect
Option (2) Chloroform and acetone — Incorrect. The H of chloroform forms a new hydrogen bond with the O of acetone, so the A–B attraction is stronger and the deviation is negative.
Option (3) Nitric acid and water — Incorrect. This mixture shows a large negative deviation and forms a maximum boiling azeotrope.
Option (4) Phenol and aniline — Incorrect. The hydrogen bond between the phenolic proton and the lone pair on the nitrogen of aniline is stronger than the hydrogen bonds between like molecules, so the deviation is negative.
NCERT (2026-27 edition):
1.5.2 Non-ideal Solutions – “… In case of positive deviation from Raoult’s law, A-B interactions are weaker than those between A-A or B-B … Mixtures of ethanol and acetone behave in this manner. In pure ethanol, molecules are hydrogen bonded. On adding acetone, its molecules get in between the host molecules and break some of the hydrogen bonds between them. Due to weakening of interactions, the solution shows positive deviation from Raoult’s law …”