In steam distillation, the mixture consisting of the organic liquid and water boils at a temperature when the sum of the vapour pressure of the liquid (p1) and that of water (p2) becomes equal to the atmospheric pressure(p) i.e.,p=p1+p2.
Since the vapor pressure of the water around the boiling point of the mixture is quite high and that of the liquid is quite low(10-15mm), therefore, the organic liquid distils at a pressure much lower than the atmospheric pressure that means, the organic liquid vaporises at a temperature much lower than its normal boiling point.
