fractional distillation

Meaning: Fractional distillation separates miscible liquids with fairly close boiling points. Vapor rises through a fractionating column packed with beads or glass indentations, where it repeatedly condenses and revaporizes. Each cycle (a theoretical plate) enriches the vapor in the more volatile component, the one with the lower boiling point and higher vapor pressure. Temperature falls up the column, so that component reaches the condenser first.

Equation: Pi = XiP°i, where Pi is the partial pressure of component i (Pa), Xi is its mole fraction in the liquid (unitless), and P°i is the vapor pressure of pure i (Pa).

On the MCAT: Passages contrast simple distillation, which suits liquids with widely different boiling points, with fractional distillation for close ones. Expect questions on which component distills first, why more plates or a longer column improves separation, and how to read a temperature versus composition diagram.

Common confusion: Separation depends on boiling point, which is set by intermolecular forces, not on density. The lower-boiling liquid comes off first. Some mixtures, such as ethanol and water, form azeotropes, so distillation cannot purify them past the azeotropic composition.