The process used to separate the substances composing a mixture; it involves a change of state, e.g. liquid to gas, and subsequent condensation.

A simple distillation apparatus consists of three parts a flask in which the mixture is heated, a condenser in which the vapor is cooled, and a vessel in which the condensed vapor, called distillate, is collected.

Upon heating, the substances with a higher boiling point remain in the flask and constitute the residue. When the substance with the lowest boiling point has been removed, the temperature can be raised and the process repeated with the substance having the next lowest boiling point. The process of obtaining portions in this way is called fractional distillation.

In destructive distillation various solid substances, such as wood, coal and oil shale, are heated out of free contact with air, and the portions driven off are collected separately. Distillation is used in refining petroleum and in preparing alcoholic beverages.