distillation

Meaning: Distillation separates liquids by differences in volatility. Heating vaporizes mostly the component with the higher vapor pressure (lower boiling point). The vapor enters a condenser, liquefies, and is collected as the distillate, while the less volatile component stays in the pot. Weaker intermolecular forces mean a lower boiling point. Simple distillation suits widely separated boiling points. Fractional distillation uses a packed column that gives repeated vaporization-condensation cycles, so it separates closer boiling points.

Equation: Ptotal = XA P°A + XB P°B, where Ptotal is the total vapor pressure (Pa), X is the mole fraction in the liquid (unitless), and P° is the vapor pressure of the pure liquid (Pa). This is Raoult's law for an ideal solution. The liquid boils when Ptotal equals the external pressure.

On the MCAT: Expect an apparatus diagram or a graph of temperature versus distillate volume. You may be asked which compound distills first (the lower boiling point), whether simple or fractional distillation is needed, or why vacuum distillation suits a heat-sensitive compound (reduced pressure lowers the boiling point).

Common confusion: Distillation separates by boiling point, not by density or solubility (that is extraction). The thermometer bulb sits at the side-arm to read vapor temperature, and condenser water enters at the bottom. Azeotropes, such as ethanol and water, boil at a fixed composition and cannot be fully separated by distillation.