Fractional Distillation

Fractional distillation is a method of separating liquids from a mixture of liquids (two or more miscible liquids). This method is widely used in industries to separate miscible liquids with boiling points that are very close to each other. For example, separating different fractions from crude oil.

Fractional distillation works because the different liquids in the mixture have different boiling points.


To carry out fractional distillation (with two liquids):

1. Heat the mixture to the temperature of the substance with the lowest boiling point (B.P).

  • Use a thermometer to track the temperature
  • The flask containing the mixture is connected to a glass column, which is called a fractionating column.

2. As you heat the mixture, the substance with the lowest B.P will evaporate more easily.

3. So, the mixture of the two different vapours rise into the fractionating column

  • The fractionating column is cold at the top and hot at the bottom. So, when the vapours of both substances reach the cooler part of the fractioning column, the substance with the higher B.P will tend to remain in liquid form. This means that it will drip back into the distilling flask.

4. The substance with the lower B.P will stay as a vapour, even at lower temperatures. So, it will rise through the column, then cool and condense in the condenser, turning into a liquid..

5. Collect the liquid in the receiving flask.

  • The substance with a higher B.P will remain in the distilling flask

Below, you can see the apparatus for fractional distillation in a lab.

Fractional distillation apparatus

This is the procedure and apparatus for carrying out fractional distillation in the lab. However, this is not as effective at carrying out fractional distillation with larger volumes of liquid.

If liquids in the mixture have similar boiling points, then it will be much more difficult to separate them. In this case, we may need to carry out multiple rounds of fractional distillation.