An azeotrope is a mixture that exhibits what behavior during boiling?

• A. It boils at a constant temperature and the vapor has the same composition as the liquid
• B. It boils at different temperatures with vapor different from liquid
• C. It cannot be vaporized
• D. It forms a solid

Answer: (A)

An azeotrope boils in a way that the vapor and liquid share the same composition at the boiling point, and the temperature remains constant during boiling. This happens because at the azeotropic composition the vapor–liquid equilibrium has y_i = x_i for each component, so the mixture behaves like a single substance. As a result, simple distillation cannot change the overall composition beyond that point—the boiling temperature stays fixed while the liquid and vapor have identical makeup. Depending on the system, this fixed boiling point can be either lower or higher than the pure components’ boiling points (minimum- or maximum-boiling azeotrope). This contrasts with non-azeotropic mixtures, where the vapor is richer in one component and the temperature changes during boiling. It’s not about being unable to vaporize or forming a solid.