Question

Absolute alcohol can be prepared from rectified spirit by distillation.
A) Vacuum
B) Azeotropic
C) Fractional
D) None of these

Answer 1

The absolute alcohol is considered as the pure state of ethanol with 100% concentration. Rectified spirit contains only 95% of alcohol and so we can obtain absolute alcohol by distillation of the rectified spirit in the presence of salt like calcium oxide or potassium carbonate.

Complete answer:
In the question, it is asked to suggest a distillation method through which we could prepare absolute alcohol from rectified spirit.
So we have to know the difference between the rectified spirit and absolute alcohol or ethanol.
Absolute ethanol or alcohol is the purest form of alcohol with 100% concentrated alcohol and it has a molecular formulae of, ${{C}_{2}}{{H}_{5}}OH$ .
Whereas rectified spirit the alcohol has a composition of 95% alcohol and the remaining 5% is the water content.
We could prepare the absolute alcohol from the rectified spirit by removing the water content, through the process of distillation, for that a step more should be incorporated in the distillation process so that the water content is removed and will yield absolute ethanol.
For the distillation process to carry out, we should compare the properties of the liquid pair in the mixture.
If we compare the chemical properties of water and ethanol, the concentration of the solution does not change even when we give heat up to its boiling point.
And such type of mixture of liquids is called an azeotropic mixture, which has the same concentration of vapour and liquid phase.
So here azeotropic distillation is used for the distillation of the rectified spirit.
Azeotropic distillation is a distillation method in which the azeotrope is broken by the addition of a third compound which results in a new lower boiling point azeotrope which is heterogeneous.
Generally benzene or toluene is added as the third component for the ethanol –water mixture. The third component added is called mass separating agent (MSA).

So the correct answer for the question is option (B).

Note: There is a chance of confusion arising between the extractive and azeotropic distillation, since in both types of distillation, separating solvents are used for the process to carry out.
But in extractive distillation we use, a specific separating solvent in every mixture, which does not form an azeotropic mixture. Hence it is a better and efficient separation technique than the azeotropic distillation.