A liquid with high boiling point decomposes on simple distillation but it can be steam distilled for its purification, because the steam distillation is used for separating substances which are volatile, insoluble in water and also has a high vapour pressure at the 100 ^oC, i.e the boiling point of water.

It is also used for purifying liquids which are temperature sensitive and it decomposes at their normal boiling point. Steam distillation is used for separating organic compounds like aromatic compounds which are mainly extracted from plant parts.