Which method separates a liquid-liquid mixture with different boiling points, such as alcohol and water?

Which method separates a liquid-liquid mixture with different boiling points, such as alcohol and water?

• A. Filtration
• B. Chromatography
• C. Crystallization
• D. Distillation

Answer: (D)

This question tests using boiling-point differences to separate components, which is accomplished by distillation. When a liquid–liquid mixture like alcohol and water is heated, the component with the lower boiling point—ethanol, around 78°C at 1 atm—vaporizes first. The vapor travels to a condenser, turns back into liquid, and is collected separately, while the higher-boiling component (water) remains mostly behind. This leverages the basic fact that different substances have different tendencies to vaporize at a given temperature. If the boiling points are close, a fractionating column can be added so the vapor undergoes repeated vaporization-condensation cycles, improving separation. Filtration wouldn’t work here because there’s no solid to trap; chromatography is for separating components based on interactions with a stationary phase, not simply separating liquids by boiling point; crystallization is for forming solids from solutions, not for separating two liquids.

This question tests using boiling-point differences to separate components, which is accomplished by distillation. When a liquid–liquid mixture like alcohol and water is heated, the component with the lower boiling point—ethanol, around 78°C at 1 atm—vaporizes first. The vapor travels to a condenser, turns back into liquid, and is collected separately, while the higher-boiling component (water) remains mostly behind. This leverages the basic fact that different substances have different tendencies to vaporize at a given temperature.

If the boiling points are close, a fractionating column can be added so the vapor undergoes repeated vaporization-condensation cycles, improving separation. Filtration wouldn’t work here because there’s no solid to trap; chromatography is for separating components based on interactions with a stationary phase, not simply separating liquids by boiling point; crystallization is for forming solids from solutions, not for separating two liquids.