Distillation: Vapor–Liquid Equilibria

Distillation separates miscible liquids with significantly different boiling points that reflect different vapor–liquid equilibrium relationships, which represent how the various components are distributed between the vapor and liquid phases.

When the mixture is heated, the first vapor bubble appears at the bubble point of the more volatile component.

The vapor is condensed to a liquid, called a distillate, and collected separately from the mixture. The early distillate is rich in the more volatile component because the less volatile component still favors the liquid phase.

The temperature needed to maintain boiling increases as the more volatile component boils off.

So, as the distillation progresses, the evaporation rate of the less volatile component increases, making the middle distillate a more even mixture. The final distillate is rich in the less volatile component.