Which statement is correct for solutions?

The correct answer is Option (4) → If \( \Delta H_{mix} \gt 0 \) and \( \Delta V_{mix} \gt 0 \), then solutions show positive deviations.

An ideal solution obeys Raoult’s law over the whole range of concentration. In it the A–B attractions are equal to the A–A and B–B attractions, so no heat is absorbed or released and there is no change in volume on mixing:

    \( \displaystyle \Delta_{mix}H = 0, \quad \Delta_{mix}V = 0 \)

When the A–B attractions are weaker than the A–A and B–B attractions, molecules escape more easily, the vapour pressure is higher than Raoult’s law predicts, and the solution shows positive deviation. Energy is needed to break the stronger A–A and B–B attractions, so heat is absorbed (\( \Delta_{mix}H \gt 0 \)), and the molecules are held less tightly, so the volume increases (\( \Delta_{mix}V \gt 0 \)). Ethanol and acetone form such a solution.

When the A–B attractions are stronger, the opposite happens: negative deviation, \( \Delta_{mix}H \lt 0 \) and \( \Delta_{mix}V \lt 0 \), as for chloroform and acetone.

Why the other options are incorrect

• Option (1) Ideal solutions do not obey Raoult's law. — Incorrect. Obeying Raoult’s law over the entire range of concentration is the very definition of an ideal solution.

• Option (2) For an ideal solution, \( \Delta H_{mix} \gt 0 \) and \( \Delta V_{mix} \gt 0 \) — Incorrect. For an ideal solution both are zero; positive values belong to a solution with positive deviation.

• Option (3) If \( \Delta H_{mix} \gt 0 \) and \( \Delta V_{mix} \gt 0 \), then solutions show negative deviations. — Incorrect. Negative deviation comes with stronger A–B attractions, so heat is released and the volume decreases: both values are negative.

NCERT (2026-27 edition):
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 …”

Non-ideal Solutions – “… In case of positive deviation from Raoult’s law, A-B interactions are weaker than those between A-A or B-B, i.e., in this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules.”