Explain why an organic liquid vapourises at a temperature below its boiling point in steam distillation.

Answer

In steam distillation, steam is passed through a mixture of organic liquid and water. The mixture will boil at a temperature at which the sum of the vapour pressures of the organic liquid an that of water becomes equal to atmospheric pressure. Therefore, the organic liquid boils when its pressure is much less than the atmospheric pressure. That is, the organic liquid vapourises at a lower temperature than its normal boiling point.