A pure compound consists of only one type of molecule. A mixture of compounds may contain a multitude of different types of molecules.

To isolate one more compounds from a mixture requires separation. Separation is purification. The basis for separation is the different properties of the components in the mixture. Some of the characteristics by which compounds can be separated are boiling points, refractive index, infrared spectrum or by comparison with a known pure sample.

Distillation is a method of separation based on the differences in boiling points of components of a mixture. There are two types of distillation. Simple or atmospheric distillation and fractional distillation.