Azeotropes

Azeotropes are constant-boiling mixtures that cannot be separated by simple distillation because the vapour and liquid have identical compositions at the azeotropic point. A minimum-boiling azeotrope boils at a temperature lower than either pure component. This happens when the total vapour pressure of the mixture is higher than predicted by Raoult's law — meaning the mixture shows positive deviation. Positive deviation arises when A–B interactions are weaker than A–A and B–B interactions, increasing vapour pressure. The classic example is ethanol–water (boiling point 78.2°C, lower than water at 100°C and ethanol at 78.4°C). Option C (maximum-boiling azeotrope) boils higher than both components and is associated with negative deviation, where A–B interactions are stronger — for example, nitric acid–water. Option A is wrong because ideal solutions do not form azeotropes. Option D: 'zeotrope' refers to a non-azeotropic mixture; it is not the correct classification here. This concept is from NCERT Chapter 2 and is tested in JEE to distinguish between types of azeotropes and their deviation basis. Plausibility check: ethanol–water azeotrope exists at 95.6% ethanol, confirming minimum-boiling behaviour with positive deviation.