Question

Why do we use fractional distillation instead of simple distillation for separating the components of air?
(A) Because fractional distillation is cheaper
(B) Because fractional distillation is easier to use
(C) Because the difference in boiling point of components are less than $20{\text{ }}K$
(D) Because the difference in melting point of components are less than $20{\text{ }}K$

Answer 1

Recall whatever you know about fractional distillation. It is only possible when the difference between the components is quite small. It consists of a more complex apparatus with a fractionating column. Example-crude oil refining. In simple distillation the difference between the components is very large.

Complete Step By Step Answer:
Fractional distillation is the process of separation of a mixture into its component parts, or fractions.
Fractional distillation is used in many areas and for many things such as petroleum refineries, petrochemical plants and chemical plants, natural gas processing and one of these is cryogenic air separation. In this technique components of air are separated and air is purified.
Every gas has its own characteristic boiling point. In fractional distillation the sample is cooled down until each component gas liquefies. The liquefied compound falls to the bottom of a collection vessel. After all of the liquid has been retrieved, cooling continues until the temperature drops to the boiling point of the next compound and it liquefies.
Now there can be a case sometimes where the two components may have boiling points very close to each other and their difference in boiling point is very less than using fractional distillation to separate those components easily. In such a case we cannot use simple distillation because simple distillation is only possible when there is a large difference in the boiling points of the components.
Thus, we use fractional distillation instead of simple distillation for separating the components of air because the differences in boiling point of components are less than $20{\text{ }}K$ .
Therefore the correct option is C.

Note:
In fractional distillation the chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize. In fractional distillation there is use of distillation or fractionate.