Components of a binary mixture of two liquids A and B were being separated by distillation. After some time separation of components stopped and composition of vapour phase became same as that of liquid phase. Both the components started coming in the distillate. Explain why this happened.

The solution in question is an example of an azeotrope. Azeotropic solutions are binary solutions which have the same composition in liquid and vapour phase and boil at a constant temperature. The components of an azeotropic solution cannot be separated by fractional distillation. An example of azeotropes is ethanol-water mixture formed after fermentation of sugars. Fractional distillation of this solution gives a solution of 95% of ethanol. Further fractional distillation does not give a higher concentration of ethanol. This composition is similar to the one in the question and is known as the azeotrope composition. In this composition, the liquid and vapour have the same composition, and no further separation occurs.