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

The correct answer is Option (3) → Ethanol and water

• An azeotrope is a binary mixture that has the same composition in the liquid and the vapour and boils at a constant temperature.

• A solution with a large positive deviation from Raoult’s law has a higher vapour pressure than expected, so it boils at a lower temperature and forms a minimum boiling azeotrope.

• The ethanol–water mixture is of this type: fractional distillation stops at about 95% ethanol by volume.

Why the other options are incorrect

Option (1) Acetone and chloroform — Incorrect. Chloroform forms a hydrogen bond with acetone, so this mixture shows negative deviation, not the positive deviation a minimum boiling azeotrope needs.

Option (2) Nitric acid and water — Incorrect. This mixture shows a large negative deviation and forms a maximum boiling azeotrope (about 68% nitric acid by mass, boiling at 393.5 K).

Option (4) Phenol and aniline — Incorrect.

• The hydrogen bond between the phenolic proton and the lone pair on the nitrogen of aniline is stronger than the hydrogen bonding in the pure liquids.

• So this mixture shows negative deviation, not the positive deviation a minimum boiling azeotrope needs.

NCERT (2026-27 edition):
1.5.2 Non-ideal Solutions – “… There are two types of azeotropes called minimum boiling azeotrope and maximum boiling azeotrope. The solutions which show a large positive deviation from Raoult’s law form minimum boiling azeotrope at a specific composition. For example, ethanol-water mixture (obtained by fermentation of sugars) on fractional distillation gives a solution containing approximately 95% by volume of ethanol.”