Ncert Exercises & Solutions

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

In steam distillation, the distillating mixture consists of steam and the vapour of organic substance. In steam distillation, the liquid boils when the sum of the vapour pressure of the organic substance (p₁) and that of steam (p₂) becomes equal to the atmospheric pressure (p) at the temperature of distillation.

p = p_{1} + p_{2}

p_{1} = p - p_{2}

Since, the vapour pressure of the organic substance is lower than p, it vaporises below its normal boiling point without decomposition e.g., aniline which normally boils at 457 K can be distilled at 371.5 K by this process.

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