The volume of mixing \((\Delta V)\) is not zero for which of the following solutions?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Ethanol and Acetone
For an ideal solution ΔmixH = 0 and ΔmixV = 0. This holds when the A–B interactions are nearly the same as the A–A and B–B interactions, as in mixtures of very similar molecules.
In pure ethanol the molecules are hydrogen bonded. Acetone molecules get in between them and break some of these hydrogen bonds. The interactions are weakened, the solution shows positive deviation from Raoult’s law, and the volume of mixing is not zero (ΔmixV > 0).
Why the other options are incorrect
Option (1) Chloroethane and Bromoethane — Incorrect. Similar haloalkanes with similar intermolecular forces; the solution is nearly ideal, ΔmixV ≈ 0.
Option (2) Benzene and Toluene — Incorrect. The textbook example of an ideal solution; ΔmixV = 0.
Option (4) n-Hexane and n-Heptane — Incorrect. Neighbouring alkanes held only by dispersion forces; nearly ideal, ΔmixV ≈ 0.
NCERT (2026-27 edition):
Ideal Solutions – “… The ideal solutions have two other important properties. The enthalpy of mixing of the pure components to form the solution is zero and the volume of mixing is also zero, i.e., ΔmixH = 0, ΔmixV = 0”
Non-ideal Solutions – “… 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 [Fig. 1.6 (a)].”