One of the oldest of all chemical techniques is distillation. An example of distillation in its simplest form is boiling water. Indeed, boiling water and then condensing it on a cold surface is one of the best methods for water purification.

With water as the solvent, the boiling point of a solution is relatively constant (slight variations can occur due to the presence of impurities at high concentrations). And while this is generally the case for any pure solvent, it is not the case for a mixture of two or more solvents. In a mixture, the boiling point is dependent upon the relative concentrations of all of the species or, more accurately, on their mole fractions.

In the case of mixed solvents, the ability of one component to vaporize can exceed the ability of any other component to vaporize. To understand this, it is necessary to consider what...