Which of the following statements is/are incorrect?
A. Azeotropes are binary mixtures of liquids having the same compositions in liquid and vapor phase and boil at a constant temperature.
B. Osmotic pressure is not a colligative property; elevation in boiling point is a colligative property.
C. Minimum boiling azeotropes show a large negative deviation from Raoult's law at specific compositions.
D. Solution of n-hexane and n-heptane show negative deviation from Raoult's law.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → B, C and D only
Solutions of two volatile liquids are compared with Raoult’s law:
Ideal solution: obeys Raoult’s law; the A–B attractions are about the same as A–A and B–B.
Positive deviation: A–B attractions are weaker, the vapour pressure is higher than expected; a large positive deviation gives a minimum boiling azeotrope.
Negative deviation: A–B attractions are stronger, the vapour pressure is lower; a large negative deviation gives a maximum boiling azeotrope.
Colligative properties depend only on the number of solute particles: relative lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure.
A. Azeotropes are binary mixtures of liquids having the same compositions in liquid and vapor phase and boil at a constant temperature. — Correct. This is the definition of an azeotrope; it cannot be separated by fractional distillation.
B. Osmotic pressure is not a colligative property; elevation in boiling point is a colligative property. — Incorrect. Osmotic pressure is a colligative property (π = CRT depends only on the concentration of particles).
C. Minimum boiling azeotropes show a large negative deviation from Raoult's law at specific compositions. — Incorrect. Minimum boiling azeotropes show a large positive deviation; a large negative deviation gives a maximum boiling azeotrope.
D. Solution of n-hexane and n-heptane show negative deviation from Raoult's law. — Incorrect. The two alkanes have very similar, weak intermolecular forces, so the mixture is nearly ideal.
NCERT (2026-27 edition):
Non-ideal Solutions – …
