Draw a diagram of bubble plate type fractionating column. When do we require such type of a column for separating two liquids. Explain the principle involved in the separation of components of a mixture of liquids by using fractionating column. What industrial applications does this process have?
If the difference in boiling points of two liquids is not much, fractional distillation is used. The techniques is, vapors of liquid mixture are passed through a fractionating column before condensation, fitted over the mouth of the round bottom flask.
Vapours of the liquid with higher boiling point condense before the vapours of the liquid with lower boiling point. The vapours rising up in the fractionating column become rich in more volatile component. Fractionating column provides many surfaces for heat exchange between ascending vapours and descending condensed liquid. The vapours become richer in low boiling component.
One of technological application of fractional distillation is to separate different fraction of crude oil in petroleum industry into various fractions like gasoline, kerosene oil, diesel oil, lubricating oil, etc.
Another application is the separation of acetone and methanol from pyroligneous acid obtained by destructive distillation of wood.
A liquid with high boiling point decomposes on simple distillation but it can be steam distilled for its purification. Explain how is it possible ?
In steam distillation, the distillating mixture consists of steam and the vapour of organic substance. In steam distillation, the liquid boils when the sum of the vapour pressure of the organic substance $\left(p_1\right)$ and that of steam $\left(p_2\right)$ becomes equal to the atmospheric pressure (p) at the temperature of distillation.
$$p=p_1+p_2 \text { or } p_1=p-p_2$$
Since, the vapour pressure of the organic substance is lower than $p$, it vaporises below its normal boiling point without decomposition e.g., aniline which normally boils at 457 K can be distilled at 371.5 K by this process.