Question

Distillation under reduced pressure is generally used to purify those liquids which
A. Have very low boiling point
B. are volatile
C. Have high boiling points and which decompose below their boiling points
D. Have a large difference in their boiling points

Answer 1

Hint: Decreasing pressure helps in decreasing the boiling point of any compound. This process of distillation under reduced pressure is known as vacuum distillation. It is done to distil a compound before it gets decomposed.

Complete step by step answer:
 -Distillation is the process of separating different components of a mixture on the basis of difference in their boiling points.
-Distillation is used for purification of alcohol, crude oil refining, desalination, etc.
-There are some components whose boiling point is very high and so to boil such compounds, it is much better to lower the pressure at which such components are boiled rather than increasing the temperature. After lowering of the pressure, the boiling and distillation process are done. This process is known as vacuum distillation.
It is a process which is performed under reduced pressure and separates compounds based on difference in their boiling points. The pressure is reduced here because it helps in decreasing the boiling point of the compound. The compounds that boil beyond their decomposition temperature at atmospheric pressure and can be easily distilled under the atmospheric pressure without being decomposed are also separated using vacuum distillation.
In short we can say that vacuum distillation is done when the boiling point of the compound or substance is too high (T > ${150}^{\circ }C$) or a compound gets decomposed at a temperature below its boiling point and we need to distil it without it getting decomposed.
This process is used for temperature sensitive materials, purification of oils in industries and also during fire refining. Also glycerol ($C{H}_2(OH)-CH(OH)-C{H}_2(OH)$) is distilled by this method since it has a boiling point of ${290}^{\circ }C$.
-Vacuum distillation is a better form of separation method because it leads to prevention of product degradation, increases yield and purity.

So, the correct option is: (C) Have high boiling points and which decompose below their boiling points.

Note:
In vacuum distillation the pressure is reduced because it is very difficult to reach such high temperatures for normal distillation and reduction in pressure decreases the boiling point of such compounds. Hence reducing pressure helps in their distillation without them being decomposed.