A mixture that cannot be separated by simple distillation is called what?

• A. Solution
• B. Mixture
• C. Azeotrope
• D. Compound

Answer: (C)

Distillation depends on differences in boiling points to separate components. An azeotrope is a special kind of mixture that boils at a single, constant composition and temperature, so the vapor has the same makeup as the liquid. Because the vapor and liquid keep the same ratio, simple distillation can't change the composition to separate the components. To break an azeotrope, you need more advanced methods, like azeotropic distillation with an entrainer or other separation techniques that alter the system.

So, this term fits best because it describes a mixture that cannot be separated by ordinary distillation. The other terms don’t apply: a solution is a general homogeneous mixture that can often be separated by distillation if components have different volatility; a mixture is too broad a category; a compound is a pure substance with chemically bonded elements.