Which of the following is an example of minimum boiling azeotrope?

The correct answer is Option (4) → Ethanol and acetone

An azeotrope is a liquid mixture that boils at a constant temperature with the same composition in the liquid and vapour phases. The type depends on the deviation from Raoult’s law:

• Large positive deviation (A–B attractions weaker than A–A and B–B): vapour pressure higher than expected, so the mixture boils at a lower temperature → minimum boiling azeotrope.

• Large negative deviation (A–B attractions stronger): vapour pressure lower, boiling point higher → maximum boiling azeotrope.

In pure ethanol the molecules are hydrogen bonded. Acetone molecules get in between them and break some of these hydrogen bonds, so the interactions weaken and the mixture shows positive deviation. So ethanol and acetone is the example of a minimum boiling mixture here.

Why the other options are incorrect

• Option (1) Phenol and aniline — Incorrect. The phenolic H forms a strong hydrogen bond with the lone pair on the N of aniline; this mixture shows negative deviation.

• Option (2) Chloroform and acetone — Incorrect. The H of \( \ce{CHCl3} \) forms a hydrogen bond with the C=O oxygen of acetone; negative deviation.

• Option (3) Nitric acid and water — Incorrect. The textbook example of a maximum boiling azeotrope (about 68% \( \ce{HNO3} \), boiling point 393.5 K), formed by large negative deviation.

NCERT (2026-27 edition):
Non-ideal Solutions – “… The cause for these deviations lie in the nature of interactions at the molecular level. In case of positive deviation from Raoult’s law, A-B interactions are weaker than those between A-A or B-B … This will increase the vapour pressure and result in positive deviation. Mixtures of ethanol and acetone behave in this manner. In pure ethanol, molecules are hydrogen bonded. On adding acetone, its molecules get in between the host molecules and break some of the hydrogen bonds between them. Due to weakening of interactions, the solution shows positive deviation from Raoult’s law.”

Non-ideal Solutions – “… Some liquids on mixing, form azeotropes which are binary mixtures having the same composition in liquid and vapour phase and boil at a constant temperature. … The solutions which show a large positive deviation from Raoult’s law form minimum boiling azeotrope at a specific composition.”