If liquids A and B form an ideal solution, which of the following is correct?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Enthalpy of mixing is zero
An ideal solution obeys Raoult’s law over the entire range of concentration. In it the A–B attractions are nearly equal to the A–A and B–B attractions, so no heat is absorbed or given out on mixing: ΔmixH = 0 (and ΔmixV = 0).
Why the other options are incorrect
Option (2) Entropy of mixing is zero — Incorrect. Mixing two liquids makes the molecules more randomly arranged, so the entropy increases: ΔmixS > 0.
Option (3) Free energy of mixing is zero — Incorrect. The liquids mix on their own, so ΔmixG = ΔmixH − TΔmixS = −TΔmixS is negative, not zero.
Option (4) Free energy of mixing and entropy of mixing are both zero — Incorrect. Both are non-zero: ΔmixS is positive and ΔmixG is negative.
NCERT (2026-27 edition):
1.5.1 Ideal Solutions – “The solutions which obey Raoult’s law over the entire range of concentration are known as 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 … It means that no heat is absorbed or evolved when the components are mixed.”