In a mixture of miscible substances, the partial pressure of a component depends on the vapor pressure of the pure component and the relative amount of the component in the mixture. This relationship is stated as Raoult's law - the partial pressure of a component in an ideal solution is equal to the vapor pressure of the pure component multiplied by its mole fraction.

In the figure above, are shown two curves representing the temperature of the distilling vapor as a function of the volume of distillate. The curve on the left represents an idealized distillation in which the lower boiling component distills completely and then the higher boiling component distills. The curve on the right represents a more empirical curve in which the distillate at the beginning of the distillation is enhanced in the amount of the lower boiling component and the distillate at the later stages of the distillation is enriched in the higher boiling component.

Without going into the mathematical explanation, the important consequence of Raoult's law is that the vapor above a boiling mixture is enriched in the lower boiling component. By carrying out a distillation carefully, it is possible to collect portions of the distillate, which are considerably enhanced in the amount of lower boiling component and higher boiling component. If the difference in the boiling points of the two components is large, a careful distillation can separate the mixture into its two components.

By careful control of temperature and by using columns designed to increase the surface area that the distilling vapors come in contact with, it is possible to make the empirical curve more closely resemble the curve on the left than the one on the right. This technique is referred to as fractional distillation.       The apparatus for a fractional distillation is similar to the apparatus for a simple distillation with an extension added to increase the path the vapor has to travel. This extension is called a fractionating column.

There are many types of fractionating columns that are used in fractional distillation. They are all similar in that the surface area, which contacts the distilling vapor, is increased. The larger the surface area contacted by the vapor, the more efficient the column is in separating the components. There are columns which are open, columns with glass indentations called Vigruex columns, and columns which are loosely packed with glass, metal or ceramic material (see figure below).

The fractionating column is often insulated to keep the temperature of the column nearly constant. If the temperature of the column fluctuates widely, it is difficult to maintain a slow, constant distillation rate. Sophisticated fractionating columns have regulated heating coils built into them. Types of fractionating columns: (a)Vigreux (b)Open (c)Packed