Which statement about azeotropes is true?

Which statement about azeotropes is true?

• A. They can be separated by simple distillation
• B. They boil at a fixed temperature with vapor having the same composition as the liquid
• C. They occur only at high pressures
• D. They are always ideal solutions

Answer: (B)

Azeotropes are mixtures that boil at a fixed temperature and produce vapor with the same composition as the liquid. At the azeotropic composition, the liquid and vapor are in identical proportions, so boiling occurs at a constant temperature and the distillate has the same composition as the original liquid. This is why simple distillation cannot separate the components beyond that point—the mixture behaves like a single substance with a fixed composition. The non-ideal interactions between components cause this constant-boiling behavior, shown by a deviation from Raoult’s law that produces a maximum or minimum in the vapor–liquid equilibrium. The remaining statements don’t fit: azeotropes aren’t limited to high pressures, they don’t require ideal solutions, and their defining feature is the equal liquid and vapor compositions at the boiling point.

Azeotropes are mixtures that boil at a fixed temperature and produce vapor with the same composition as the liquid. At the azeotropic composition, the liquid and vapor are in identical proportions, so boiling occurs at a constant temperature and the distillate has the same composition as the original liquid. This is why simple distillation cannot separate the components beyond that point—the mixture behaves like a single substance with a fixed composition. The non-ideal interactions between components cause this constant-boiling behavior, shown by a deviation from Raoult’s law that produces a maximum or minimum in the vapor–liquid equilibrium. The remaining statements don’t fit: azeotropes aren’t limited to high pressures, they don’t require ideal solutions, and their defining feature is the equal liquid and vapor compositions at the boiling point.