Which is not true for non-ideal solutions?

The correct answer is Option (4) → obey Raoult’s law

A non-ideal solution is one that does not obey Raoult’s law over the entire range of concentration. Its vapour pressure is higher (positive deviation) or lower (negative deviation) than Raoult’s law predicts, because the A–B interactions differ from the A–A and B–B interactions. For the same reason, mixing is accompanied by a heat change and a volume change:

    Δ_mixH ≠ 0 and Δ_mixV ≠ 0

• Option (1) ΔH_mix ≠ 0 — True for a non-ideal solution. Heat is absorbed (positive deviation) or released (negative deviation) on mixing.

• Option (2) Δ_mixV ≠ 0 — True for a non-ideal solution. The volume of the solution is larger or smaller than the sum of the volumes of the components.

• Option (3) do not obey Raoult’s law — True for a non-ideal solution. This is the definition of a non-ideal solution.

• Option (4) obey Raoult’s law — Not true for a non-ideal solution. Obeying Raoult’s law over the whole range of concentration is the mark of an ideal solution.

NCERT (2026-27 edition):
Ideal and Non-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, i.e., Δ_mixH = 0, Δ_mixV = 0. … When a solution does not obey Raoult’s law over the entire range of concentration, then it is called non-ideal solution.”