Question

Absolute alcohol can be prepared from rectified spirit by:
A. Azeotropic distillation with benzene.
B. Fractional distillation.
C. Keeping over fresh ${\text{CaO}}$ for a few hours and then distilling.
D. Distillation under reduced pressure.

Answer 1

Absolute alcohol is the pure alcohol, i.e. $100\%$ alcohol. Rectified spirit is a mixture of $95.5\%$ ethanol and $4.5\%$ water. In the laboratory, it is prepared by a different method. Absolute alcohol contains $1\%$ gas. It is a continuous boiling mixture.

Formula used:

Complete answer: or Complete step by step answer:
Absolute alcohol is also known as anhydrous alcohol. Absolute alcohol cannot be separated by fractional distillation. Fractional distillation is the process in which vaporization of liquid mixture gives rise to a mixture of constituents from which the desired component is separated into pure form. Since the rectified spirit contains ethanol and water, it cannot be separated using a fractional distillation method.
Rectified spirit is a mixture which is known as azeotropic mixture. Distillation is a process in which a mixture of two liquids has different boiling points. Absolute alcohol can be prepared by the distillation of rectified spirit in the presence of ${\text{CaO}}$. This removes water content and then contacts anhydrous ${\text{CuS}}{{\text{O}}_4}$ which can remove the remaining water contents. But this has occurred on a small scale. For large scale preparation, this method cannot be used.
Thus it can also be separated using azeotropic distillation of benzene. It is also a distillation method in which azeotropes are separated.
Hence we can say that absolute alcohol can be prepared from a rectified spirit by azeotropic distillation and keeping over fresh ${\text{CaO}}$ for a few hours and then distilling.

So, the correct answer is Option A,C .

Note:
Azeotropic distillation refers to the specific technique of adding another component to generate a new, lower boiling azeotrope that is heterogenous, i.e. two immiscible liquids. Instead of benzene, hexane, cyclohexane, pentane, diethyl ether and acetone can also be used.