Distillation is a process where a mixture made of two or more liquids (called "components") with different boiling points can be separated from each other. The mixture is heated until one of the components boils (turns to a vapor). The vapor is then fed into a condenser, which cools the vapor and changes it back into a liquid that is called "distillate". What remains in the original container is called the "residue". This is a physical separation process and not a chemical reaction. Fractional distillation (using a distillation column with more than two outlets) can be used to improve the separation. An oil refinery uses fractional distillation to purify crude oil, separating several different liquids for different uses. 