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 maximum. 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.
Test Your Understanding: Azeotropic Mixtures
1. Why can the individual components of a constant boiling azeotropic mixture not be separated via simple fractional distillation?
- (A) Both components react chemically to form a single solid compound.
- (B) The liquid phase and the vapor phase have the exact same composition at that temperature.
- (C) The two liquids become completely immiscible at the boiling threshold.
- (D) One component decomposes entirely when vaporized.
View Answer
Explanation: Fractional distillation depends on a difference in composition between the liquid and vapor phases. Because an azeotrope's liquid mixture translates into a vapor phase with the exact same component ratios, boiling it changes nothing about its purity levels.
2. A mixture of Ethanol and Water forms a minimum boiling azeotrope. What type of deviation from Raoult's law does this mixture represent?
- (A) Zero deviation (Ideal Solution)
- (B) Negative deviation
- (C) Positive deviation
- (D) Extreme temperature deviation
View Answer
Explanation: Systems that exhibit positive deviations show higher vapor pressures than predicted by Raoult's law due to weaker intermolecular forces between the mixed components. A maximum vapor pressure point naturally yields a minimum boiling temperature.
3. Which of the following is an example of a Maximum Boiling Azeotrope?
- (A) Ethanol + Water
- (B) Water + HCl
- (C) Chloroform + Water
- (D) Acetone + Methanol
View Answer
Explanation: A mixture of nitric acid/water or hydrochloric acid/water experiences strong intercomponent attraction (negative deviation), bringing vapor pressure down and pulling the final systemic boiling limit to a maximum point.