Which one of the statements given below, concerning properties of solutions, describes a colligative effect?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Vapour pressure of pure benzene decreases on adding naphthalene
A colligative property arises when a non-volatile solute is added to a volatile solvent, and it depends only on the number of solute particles, not on their nature.
Naphthalene is a non-volatile solid; its molecules take up part of the surface of benzene, fewer benzene molecules escape, and the vapour pressure falls. This lowering of vapour pressure is a colligative effect.
Why the other options are incorrect
Option (1) Boiling point of pure water decreases on adding ethanol — Incorrect. Ethanol is itself volatile, so it adds its own vapour pressure; the ethanol–water mixture shows positive deviation from Raoult’s law. This is not a colligative effect.
Option (2) Vapour pressure of pure water decreases on adding nitric acid — Incorrect. Nitric acid is a volatile liquid; nitric acid–water shows large negative deviation from Raoult’s law because of strong attractions between the two liquids, which depends on their nature.
Option (4) Boiling point of pure benzene increases on adding toluene — Incorrect. Toluene is also volatile; benzene and toluene form a nearly ideal solution of two volatile liquids, and its boiling point lies between theirs.
NCERT (2026-27 edition):
1.6 Colligative Properties and Determination of Molar Mass – “We have learnt in Section 1.4.3 that the vapour pressure of solution decreases when a non-volatile solute is added to a volatile solvent. … All these properties depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution.”