Comprehension:
Solutions
Let us consider a binary solution of two volatile liquids and denote the two components as 1 and 2. When taken in a closed vessel, both the components would evaporate and eventually an equilibrium would be established between vapour phase and the liquid phase. Let the total vapour pressure at this stage be ptotal and p1 and p2 be the partial vapour pressures of the two components 1 and 2 respectively. These partial pressures are related to the mole fractions x1 and x2 of the two components 1 and 2 respectively. The French chemist, Francois Marie Raoult (1886) gave the quantitative relationship between them. The relationship is known as the Raoult's law which states that for a solution of volatile liquids the partial vapour pressure of each component of the solution is directly proportional to its mole fraction (x1) present in solution.
The change in enthalpy of mixing and the volume of mixing for the ideal solution which obeys Raoult's law will be
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Δmix H = 0, Δmix V = 0
For an ideal solution, mixing the components gives no heat change and no volume change.
Why the other options are incorrect
Option (1) Δmix H ≠ 0, Δmix V ≠ 0 — Incorrect. This describes a non-ideal solution (positive values for positive deviation, negative values for negative deviation).
Option (2) Δmix H ≠ 0, Δmix V = 0 — Incorrect. ΔmixV is zero, but ΔmixH is also zero for an ideal solution; a non-zero ΔmixH means unequal A–B and A–A/B–B attractions, i.e. a non-ideal solution.
Option (4) Δmix H = 0, Δmix V ≠ 0 — Incorrect. ΔmixH is zero, but ΔmixV is also zero for an ideal solution; a non-zero ΔmixV means unequal A–B and A–A/B–B attractions, i.e. a non-ideal solution.
NCERT (2026-27 edition):
Ideal Solutions
