Why is it not possible to obtain pure ethanol by fractional distillation? What general name is given to binary mixtures which show deviation from Raoult’s law and whose components cannot be separated by fractional distillation?

How many types of such mixtures are there?

1. Various liquids on mixing form azeotropes, which are binary mixtures which have the same composition in liquid as well as vapour phase and boil at a constant temperature. These mixtures cannot be separated into their individual components by fractional distillation.

2. Let us consider mixtures of ethanol and water. The lowest boiling point value of this mixture occurs at 95.6% concentration by mass of ethanol in this solution. The boiling point of this solution is 78.2°C, while that of pure ethanol is 78.5°C and of pure water is 100°C. The 0.3°C difference is important when it comes to separation of ethanol water mixtures. A graph showing a vapour composition curve of ethanol and water explains this further.

3. Boiling of any ethanol water mixture above 0% and below 95.6% mass will give a vapour of higher concentration of ethanol. Condensing and reboiling that mixture will finally lead to 95.6% ethanol. Reboiling further gives vapour and condensate of the same concentration, as graphically, this is where the liquid curve and the vapour curve meet. It is impossible to obtain pure ethanol from this mixture. It boils as a pure liquid with a constant boiling point. This is an azeotropic mixture or a constant boiling mixture.

4. Azeotropes can either have a lower boiling point or a higher boiling point than their constituents. The various types of azeotropes are as follows.

5. (i) Heterogeneous and homogeneous azeotropes: If the constituents of the azeotrope are not completely miscible, they are called heterogeneous azeotropes. Homogeneous azeotropes are solutions with completely miscible components. (ii) Positive and Negative azeotropes: An azeotropic mixture which has a lower boiling point than its components is termed as a positive azeotrope. These mixtures show positive deviation from the vapour pressure calculated by Raoult’s Law. They form minimum boiling azeotropes. The ethanol water mixture mentioned above is an example of this type. Negative azeotropes are mixtures with higher boiling points than their constituent boiling points. They show large negative deviation from vapour pressure calculated by Raoult’s law. They form maximum boiling azeotropes. An example of negative azeotrope is nitric acid and water. This azeotrope has the approximate composition, 68% nitric acid and 32% water by mass, with a boiling point of 393.5 K.