Distillation has been used widely to separate volatile components from nonvolatile compounds. The underlying mechanism of distillation is the differences in volatility between individual components. With sufficient heat applied, a gas phase is formed from the liquid solution. The liquid product is subsequently condensed from the gas phase by removal of the heat. Therefore, heat is used as the separating agent during distillation. Feed material to the distillation apparatus can be liquid and/or vapor, and the final product may consist of liquid and vapor. A typical apparatus for simple distillation used in chemistry laboratories is one in which the still pot can be heated with a water, steam, or oil bath. When liquids tend to decompose or react with oxygen during the course of distillation, the working pressure can be reduced to lower the boiling points of the substances and hence the temperature of the distillation process.
In general, distillation can be carried out either with or without reflux involved. For the case of single-stage differential distillation, the liquid mixture is heated to form a vapor that is in equilibrium with the residual liquid. The vapor is then condensed and removed from the system without any liquid allowed to return to the still pot. This vapor is richer in the more volatile component than the liquid removed as the bottom product at the end of the process. However, when products of much higher purity are desired, part of the condensate has to be brought into contact with the vapor on its way to the condenser and recycled to the still pot. This procedure can be repeated for many times to increase the degree of separation in the original mixture. Such a process is normally called "rectification."