Azeotropic Mixture - Practice Questions & MCQ

An azeotropic mixture is a solution of two liquids  having a certain composition in which both the gas phase and the liquid phase composition are the same i.e. . These solutions distill over without changes in composition and hence, these cannot be separated by distillation. These solutions are formed by non-ideal solutions which show a large deviation from ideality.

These solutions boil at one particular temperature like a pure liquid and distills over in the same composition and hence are also referred to as constant boiling mixtures.

Types of Azeotropic Mixtures

It is of the following types:

1. Minimum Boiling Azeotropes

   Non-ideal solutions showing large positive deviation from Raoult's law form minimum boiling azeotropes which boil at temperature lower than boiling point of either of the components 'A' or 'B'. 

   In the figure given below, the point M represents the azeotropic composition. At this point, vapour pressure is maximum and therefore the solution has lowest boiling point.

   

    

   e.g., Ethanol water mixture on fractional distillation gives a solution containing approximately 95 % by volume of ethanol. Once this composition is achieved, no further separation occurs.

    

2. Maximum Boiling Azeotropes

   Non-ideal solutions showing large negative deviation from Raoult's law form maximum boiling azeotropes which boil at temperature higher than the boiling point of either of the components 'A' or 'B'. 

   In the figure given below, the point B represents the azeotropic composition. At this point, vapour pressure is minimum and therefore the solution has the lowest boiling point.

   

   e.g Nitric acid and water is an example of this class of azeotrope. This azeotrope has the approximate composition, 68% nitric acid and 32% water by mass

We have two liquids A and B and their vapour pressures are represented as P_A and P_B.

According to Raoult's law, we know:

      

Now, according to Dalton's law of partial pressure, we have:

Thus, on combining equations (i) with (iii) and (ii) with (iv), we get: