What is a fractionating column? Explain its advantage.

When the boiling points of the two liquids are quite close (difference less than 20-25°C) the separation is not successfully affected by using simple distillation process but is affected by fitting the flask with a fractionating column.
A fractionating column is a long tube provided with obstructions or packed with glass beads. The obstructions provided slows down the passage of vapours upwards and that of the liquid downwards.

Fig. Fractionating columns. 

Advantages. Suppose you have a mixture of liquids (say A with boiling point 80°C and B with boiling point 90°C). Set up the apparatus as shown in Fig. 2.9. On heating the flask the vapours obtained consist more of A and less of B. As the vapours rise up the fractionating column, they condense partially and the condensed liquid flows down. Vapours of B having higher boiling point will condense more rapidly than the vapours of A having lower boiling point. As a result the vapours moving up get richer in A. The condensed liquid flowing down meets the ascending stream of vapours and in the process takes away more of high boiling liquid B. The process is


Fig. Fractional distillation.

repeated throughout the length of fractionating column. Consequently, the vapours which escape from the top of the fractionating column into the condenser consist of almost pure A. Thus the distillate received is pure A whereas the liquid left behind in the flask is rich in B. Pure B can then be obtained by simple distillation. The process of fractional distillation with fractions A and B are repeated if the boiling point difference is very low.