Benzoic acid is an organic compound. Its crude sample can be purified by crystallisation from hot water. What characteristic differences in the properties of benzoic acid and the impurity make this process of purification suitable?
Benzoic acid can be purified by hot water because of following characteristics
(i) Benzoic acid is more soluble in hot water and less soluble in cold water.
(ii) Impurities present in benzoic acid are either insoluble in water or more soluble in water to such an extent that they remain in solution as the mother liquor upon crystallisation.
Two liquids $(A)$ and $(B)$ can be separated by the method of fractional distillation. The boiling point of liquid $(A)$ is less than boiling point of liquid $(B)$. Which of the liquids do you expect to come out first in the distillate ? Explain.
If the difference in boiling points of two liquids is not much, fractional distillation is used to separate them. In this technique, fractionating column is fitted over the mouth of the round bottom flask.
When vapours of a liquid mixture are passed through a fractionating column, the vapours of the low boiling liquid $(A)$ will move up while those of the high boiling liquid will condense and fall back into the flask. Therefore, liquid $(A)$ with low boiling point will distill first.
You have a mixture of three liquids $A, B$ and $C$. There is a large difference in the boiling points of $A$ and rest of the two liquids i.e., $B$ and $C$. Boiling point of liquids $B$ and $C$ are quite close. Liquid $A$ boils at a higher temperature than $B$ and $C$ and boiling point $B$ is lower than $C$. How will you separate the components of the mixture. Draw a diagram showing set up of the apparatus for the process.
The boiling points are in the order of $A>C>B$.
Liquid $A$ can be separated from rest of the mixture of liquid $B$ and $C$ by simple distillation $B$ and $C$ can be separated by fractional distillation.
Due to the fact that boiling point of $A$ is much higher than those of liquids $B$ and $C$. This can be done by using apparatus as shown in figure (I). As the boiling points of liquid $(B)$ and $(C)$ are quite close but much lower than that of $A$, hence, mixture of liquids $(B)$ and $(C)$ will distill together leaving behind liquid $(A)$.
On further heating liquid $(A)$ will distill over. Now, place the mixtures of liquid $(B)$ and $(C)$ in a flask fitted with fractionating column as illustrated in figure (II). On fractional distillation, liquid $(B)$ will distill over first and then liquid $(C)$ as former possess lower boiling point than that of later.
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.