ANS:D - azeotropic

Solutions that distil without change in composition are called azeotropic. In an azeotropic mixture, the liquid and vapor phases have the same composition, and thus, during distillation, the vapor that is produced has the same composition as the original liquid mixture. As a result, no separation of components occurs during distillation, making it impossible to further purify the mixture by simple distillation. Azeotropic mixtures are common in solutions where the components form a specific ratio that results in a constant boiling point at a particular composition. Examples include ethanol-water mixtures, where the azeotropic composition is around 95% ethanol and 5% water, and hydrochloric acid-water mixtures, where the azeotropic composition is around 20.2% hydrochloric acid and 79.8% water.