Process of simple distillation can be used to separate the constituents from a liquid mixture differing the their boiling points by `25^(@)C` or more. However, fractional distillation is effective if the difference in boiling points is less than `25^(@)C`. How will you explain this ?

Explanation

In simple distillation, only the low boiling liquid will distil while high boiling liquid will remain in the distillation flask. Thus, separation can be done if the difference in the boiling points of the liquids is `25^(@)C` or more. In the second case, both the liquids will distil simultaneously. The distillate will contain the fractions of both the liquids. If a fractionating column is used, the vapours of high boiling liquid will also rise into the column along with the low boiling liquid. But they will condense first releasing energy (called latent heat of condensation) and fall back in the distillation flask as a liquid. This energy will be absorbed by the vapours of the low boiling liquid which will remain in the vapour state. It will get distilled while the high boiling liquid unable to get distilled, remain in the distillation flask only. In this way, separation can be done. Thus, the role of fractionating column is to put obstructions in the path of the vapours of the liquids that are rising upwards.