Rectified spirit contains about `95%` alcohol (b.p. `351 K`) and `5%` water (b.p. `373 K`). The two components can be separated by

Options

A. simple distillation
B. azeotropic distillation
C. fractional distillation
D. steam distillation

Answer

B

Explanation

The two components cannot be separated from the mixture by distillation no matter how efficient the fractionating column used is, even though their boiling points differ by `22 K`. At the given composition, alcohol and water form a constant boiling mixture called azeotrope, which distils like a pure liquid at a certain constant temperature. To remove water from such a mixture (binary azeotrope), we perform fractional distillation using a suitable volatile solvent. The process is called azeotropic distillation.
In the present case, the process is carried out with a suitable amount of benzene. The first fraction obtained at `331.8 K` is a ternary azeotrope containing mostly water, some alcohol, and some benzene. The second fraction distilled out at `341.2 K` is a binary azeotrope containing some alcohol and remaining benzene. The third fraction distilling at `351 K` is absolute alcohol.