What is Azeotrope or Azeotropic Mixture?

Mixtures of liquids which boils at constant temperature like a pure liquid such that the distillate has the same composition as that of liquid mixture, called constant boiling mixtures or azeotropic mixtures or simply azeotropes. Obviously, the components of an azeotrope cannot be separated by fractional distillation.


These azeotropes are of two types-

Minimum Boiling (Point) Azeotropes

These azeotropes are formed by liquid pairs which show positive deviations from ideal behaviour. Such an azeotrope corresponds to an intermediate composition for which the total vapor pressure is the highest and hence the boiling point is the lowest. Such azeotropes have boiling points lower than either of the pure components. Ethyl alcohol-water is an example of this type of azeotrope.

Maximum Boiling (Point) Azeotropes

These azeotropes are formed by liquid pairs which show negative deviations from ideal behaviour (Raoult's law). Such an azeotrope corresponds to an intermediate composition for which the total vapor pressure is the minimum and hence the boiling point is the minimum. Such azeotropes have boiling points higher than either of the pure components. Water-HCl is an example of this type of azeotrope.

Other Azeotropic mixtures are-

Homogeneous Azeotropes

If the constituents of an azeotropic mixture are completely miscible in all proportions with each other are considered to be a homogeneous azeotrope. Any amount of ethanol can be mixed with water to form a homogeneous azeotropic mixture.

Heterogeneous Azeotrope

If the azeotropes are present as a constituent in several mixtures and if these mixtures are not entirely miscible are considered as heterogeneous azeotropes. These heterogeneous azeotropes are incompletely miscible and form two liquid phases. Chloroform and water, benzene, and water are example of heterogeneous azeotropes.

Binary Azeotrope

Azeotropes that contain precisely two constituents as their components are called binary azeotropes. Mixture of diethyl ether (33%) and halothane (66%) is an example of binary azeotrope. This azeotropic mixture is most commonly used in medical fields as anaesthesia.

Ternary Azeotrope

Azeotropes which contain three constituents as their components are called ternary azeotropes. Mixture of acetone, methanol, and chloroform is an example of ternary azeotrope.

Application of Azeotropes

Some Important applications of azeotropes are given below-
1. It is mainly used as a standard in several fields, such as testing chromatographs, detectors, and columns.
2. It is also utilized to separate the components of azeotropic mixtures.
3. It is also employed to extract essential oils from plant materials.