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 `(p_(1))` and that of water `(p_(2))` becomes equal to the atmospheric pressure (p), i.e., `p = p_(1) + p_(2)`.
Since the vapour pressure of water around the boiling point of the mixture is quite high and that of liquid is quite low (10 -15mm), therefore, the organic liquid at a pressure much lower than the atmospheric pressure. In other words, the organic liquid vapourises at a temperature much lower than its normal boiling point.
