Why is the boiling point of rectified spirit lower than alcohol absolute? The boiling point of rectified spirit is ($78.1~\mathrm{^\circ C}$) while that of absolute alcohol is ($78.3~\mathrm{^\circ C}$). I know its too small, but shouldn't the addition of impurities increase the boiling point? 

What qualifies as an impurity? Is it that solids like salt only can be called an impurity? — which increase the boiling point of water?
Ethanol and water have weaker intermolecular interactions than water with water and ethanol with ethanol. For this reason ethanol and water molecules can more easily escape from the liquid mixture, called azeotrope, to the gaseous phase than it is the case for the pure alcohol. Consequently the azeotrope has a lower boiling point.

What is considered as an impurity depends on your use case. E.g. 96% pure ethanol may qualify as extra fine spirit.