simple distillation

Meaning: Simple distillation separates liquids by boiling point. Heating vaporizes mainly the more volatile component (lower boiling point, higher vapor pressure). Its vapor enters a condenser, cools, and drips into the receiver as distillate, while the less volatile component stays in the flask. It works best when boiling points differ widely (about 25 °C or more is a common rule of thumb) or when a liquid is separated from a nonvolatile solute.

Equation: Ptotal = XA P°A + XB P°B, where X is the mole fraction in the liquid (unitless) and P° is the pure-component vapor pressure (Pa). A liquid boils when its vapor pressure equals the external pressure.

On the MCAT: Expect questions on which compound distills first, what a plateau on a temperature-versus-volume graph means, thermometer placement (bulb level with the side arm of the distillation head), and cooling water entering the condenser at the bottom and leaving at the top. Closer boiling points call for fractional distillation. Heat-sensitive or high-boiling liquids call for vacuum distillation, since lower pressure lowers the boiling point.

Common confusion: Distillation separates by volatility, not directly by molar mass or density. Hydrogen bonding raises a boiling point, so a smaller alcohol can boil higher than a larger ether. Also, simple distillation cannot fully separate an azeotrope such as ethanol and water.