Answer:
Fractional distillation is used for the separation and purification
of organic liquids from non-volatile impurities or for separating two or more
volatile liquids from a liquid mixture which have boiling points close to each
other.
Since, in this process, the
distillate is collected in fractions under different temperatures, it is known
as fractional distillation.
When the liquids present in the
mixture have their boiling points close to each other, the separation into pure
liquids is difficult. The fractions collected are always contaminated.
The fractions are further
purified by repeated distillations. To decrease the number of distillations,
the separation is done by fitting the distillation flask with a fractionating
column*which in turn is connected to the condenser. Various types of fractionating
columns are given in Fig.
Liquids forming a constant
boiling mixture (azeotropic mixture) cannot be separated by this method.
Fractional distillation is used these days in industry, especially, in the distillation
of petroleum, coal-tar and crude alcohol. A mixture of methanol (b.pt. 338K)
and propanone (b.pt.330K) or a mixture of benzene and toluene may be separated by
fractional distillation.
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