Reflux is a method of applying constant heat to a mixture without losing any of the liquid to evaporation. The vessel containing the chemical mixture is placed in a second vessel filled with water or oil. It is then attached to a condenser, which cools vapors back into liquid.
Reflux allows chemists to observe chemical reactions that require a constant temperature, controlled conditions and substantial time. It can also be used to separate the components of chemical compounds. Reflux is used on an industrial scale in oil refineries, chemical manufacturing plants and natural gas processing plants, and in the manufacture of high-quality alcoholic beverages.