Azeotropic mixtures are mixtures of two or more liquids with similar boiling points and a similar composition in their vapour phase. Azeotropic mixtures do not obey Raoult’s Law. 

Raoult’s law tells us that a solution’s vapour pressure is always equal to the sum of the vapour pressure of the volatile components of that particular solution, based on the mole fraction of the solution’s components. The Azeotropic mixtures can be categorised into two types, depending on the extent and type of deviation of the azeotropic solutions from Raoult’s law. 

What is an Azeotropic Mixture?

• Azeotropic mixture is a mixture of two or more liquids with a similar boiling point and a similar composition in their vapour phase. 
• Azeotropic mixture either has a higher or lower boiling point than its components.
• Components of an azeotropic solution cannot be separated by simple distillation as their boiling points have the same value.
• Azeotropic mixture, where its constituents are completely miscible, is known as homogeneous azeotropic mixtures.
• Azeotropic mixture, where its constituents are not completely miscible, is known as heterogeneous azeotropic mixtures.
• The binary azeotropic mixture has two constituents or liquids, whereas the ternary azeotropic mixture has more than three liquids. 

Azeotropic Mixture Notes

Azeotropic mixtures generally consist of two constituents, one solute and one solvent. The solute and solvent are in a liquid phase. The composition of the solute and solvent is in a fixed proportion. Azeotropic mixtures have a fixed boiling point.

Negative Azeotropic Mixture

Also known as minimum boiling azeotropic mixtures, the mixture has a boiling point higher than its constituents. For example, a mixture of hydrogen chloride and water boils at 110℃ whereas individually, hydrogen chloride boils at -84℃ and water at 100℃.

Positive Azeotropic Mixture

Also known as maximum boiling azeotropic mixtures, the mixture has a lower boiling point than its constituents. For example, ethanol and water. Ethanol and water, together, boil at 78.2℃, whereas ethanol boils at 78.4℃ and water boils at 100℃.

Separation Process

• Distillation cannot separate an azeotropic mixture into its constituents as constituents in the vapour phase have a similar proportion to that of the normal liquid phase azeotrope.
• Constituents of an azeotropic mixture boil at the same temperature, making it difficult to separate the mixture into its constituents.
• Special distillation techniques like azeotropic distillation can separate the azeotropic mixtures.
• In azeotropic distillation, the mixture is broken down using another volatile component, commonly known as the entrainer or the mass separating agent.
• This mass separating agent is added to the azeotropic mixture to form a new lower-boiling azeotropic mixture. This lower-boiling mixture enters a separator and splits into two different phases, which can be easily separated.

Applications

• Azeotropic substances are used in gas chromatography to minimise the error due to changes in the composition of the mixture. For example, cyclohexane and benzene mixture.
• Minimum boiling azeotropic mixtures are of great importance to chemists.

Conclusion

Azeotropic mixtures are mixtures of two or more liquids with similar boiling points and a similar composition in their vapour phase. Azeotropic mixtures have either higher or lower boiling points than any of their components. Components of an azeotropic solution cannot be separated by simple distillation as they have the same boiling points.

Raoult’s law tells us that the vapour pressure of a solution is always equal to the sum of the vapour pressure of the volatile components of that particular solution based on the mole fraction of the solution’s components. Azeotropic mixtures do not obey Raoult’s Law. 

Based on deviation from Raoult’s law, the azeotropic mixtures are categorised into two different types: a minimum boiling azeotropic mixture and a maximum boiling azeotropic mixture. Azeotropic mixtures are used in gas chromatography, and they are very useful to chemists for various chemical reactions.