Fractional Distillation

Fractional distillation is a method of separating two or more miscible liquids from a mixture. This method is widely used in industries to separate liquids with close boiling points. 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 rises 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 fractionating 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 remain as a vapour, even at cooler temperatures. 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, is the apparatus for fractional distillation in a lab.

A diagram of fractional distillation with labelled apparatus.
Fractional distillation apparatus

This procedure and apparatus are suitable for carrying out fractional distillation in the lab. However, they are not as effective for 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.

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