Distillation is a process used for purification of organic compounds. The distillation process is based on physical property of compounds, such as normal boiling point. Distillation process consists of mixture heating and then condensing. Since the components of mixture have different boiling points, hence, by heating, mixtures can be separated into its components. The boiling point is the temperature at which the external pressure acting on the liquid surface becomes equal to the vapor pressure of the liquid phase of a compound. Usually, the atmospheric pressure is considered as the external pressure.
Hence, if a liquid mixture is heated in an open flask, the vapor pressure of the liquid is increasing as the temperature of the liquid increases. When the atmospheric pressure will be equal to the vapor pressure, the liquid will boil. Since the compounds of mixture have different boiling points, distillation process uses this physical property to facilitate the separation of compounds from a mixture.
The first phase in distillation is registered when the heated substance passes from condensed state to gaseous state. This phase is called vaporization for liquid mixtures and sublimation for solid mixtures.