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Distillation under reduced pressure and Steam distillation - (Concept)

Distillation under reduced pressure or vacuum distillation is a type of distillation process that is used for the purification of high boiling liquids and liquids which decompose at or below their normal boiling point.

If the pressure on the surface of the liquid is reduced by suction pump or vacuum pump, the liquid boils at lower temperature without decomposition. e.g., glycerol (b.pt. 563 K) can be distilled at 453 K under 12 mm Hg pressure without decomposition.

Raw juice in sugar factories is generally concentrated by vacuum distillation.

Steam distillation process is used for the seperation and purification of organic compounds (solid or liquid) which are volatile in steam, immiscible with water, possess a high vapour pressure of about 10-15 mm Hg at 373 K and contain non-volatile impurities.

In this process steam is passed through the organic mixture to be distilled so that distilling mixture consists of steam and volatile organic compound, which follows that

Atmospheric pressure = vapour pressure of organic substance + vapour pressure of steam.

From the above relation it can be interpretated that organic compounds distil below its normal boiling point without decomposition. For example,
(1) Aniline can be distilled at 371.5 K against its normal boiling point 457 K.  
(2) o-Nitrophenol can be seperated from p-Nitrophenol, since o-Nitrophenol is volatile in steam.

Exam Chapter
JEE MAIN Purification and Characterisation of Organic Compounds
Chemistry Part II Textbook for Class XI
Page No. : 358
Line : 29

This method is used to purify liquids having very high boiling points and those, which decompose at or below their boiling points.


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