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.
Which of the following pairs of liquids can be separated into the pure components by fractional distillation?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Benzene–toluene
Fractional distillation separates two liquids because the vapour is richer in the more volatile one. It fails for an azeotrope, whose liquid and vapour have the same composition, so the mixture boils at a constant temperature without changing.
Benzene and toluene have nearly equal A–A, B–B and A–B attractions, so they form a nearly ideal solution. An ideal solution forms no azeotrope, so repeated distillation gives pure benzene and pure toluene.
Why the other options are incorrect
Option (2) Water–HNO3 — Incorrect. It shows a large negative deviation and forms a maximum boiling azeotrope (about 68% nitric acid by mass), which distils unchanged.
Option (3) Water–HCl — Incorrect. Water and hydrogen chloride also form a maximum boiling azeotrope, so distillation stops at that composition.
Option (4) Water–C2H5OH — Incorrect. Fractional distillation gives only about 95% ethanol by volume, the minimum boiling azeotrope; no further separation occurs.
NCERT (2026-27 edition):
1.5.1 Ideal Solutions – “… If the intermolecular attractive forces between the A-A and B-B are nearly equal to those between A-B, this leads to the formation of ideal solution. A perfectly ideal solution is rare but some solutions are nearly ideal in behaviour. Solution of n-hexane and n-heptane, bromoethane and chloroethane, benzene and toluene, etc. fall into this category.”
1.5.2 Non-ideal Solutions – “Some liquids on mixing, form azeotropes which are binary mixtures having the same composition in liquid and vapour phase and boil at a constant temperature. In such cases, it is not possible to separate the components by fractional distillation.”