Question

A liquid with high boiling point decomposes on simple distillation but it can be steam distilled for its purification. Explain how is it possible?

Answer 1

A liquid with high boiling point which decomposes on simple distillation would be difficult to separate by a simple distillation process. This type of liquids can be purified using the steam distillation process.

Steam distillation:

• Steam distillation technique is applied to separate substances which are steam volatile and are immiscible with water. It is often used to purify liquids that decompose at their usual boiling point.
• In steam distillation, steam from a steam generator is passed through a heated flask containing the liquid to be distilled. The mixture of steam and the volatile organic compound is condensed and collected. The compound is later separated from water using a separating funnel.

In steam distillation, the liquid boils when the sum of vapour pressures due to the organic liquid $\left(p_1\right)$ and that due to water $\left(p_2\right)$ becomes equal to the atmospheric pressure $\left(p\right)$, i.e., $p=p_1+p_2$, since $\left(p_1\right)$ is lower than $\left(p_2\right)$, the organic liquid vaporises at lower temperature than its boiling point. Thus, distillation takes place at lower temperature.