Fractional Distillation

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More Lessons for IGCSE Chemistry

Fractional distillation is a method for separating liquids with different boiling points.

For example, liquid ethanol can be separated from a mixture of ethanol and water by fractional distillation. This method works because the liquids in the mixture have different boiling points. When the mixture is heated, one liquid evaporates before the other.

In this experiment, we will use fractional distillation to separate a mixture of ethanol and water.

The boiling point of ethanol is 78°C and the boiling point of water is 100°C.

How to set up the apparatus for fractional distillation?

How to use fractional distillation to separate ethanol and water?

1. Set up the apparatus.
2. Heat the mixture until the temperature at the top of the fractionating column is 78°C (which is the boiling point of ethanol). The ethanol vapour passes into the condenser, gets cooled and collects in a beaker kept at the other end of the condenser. The temperature is maintained at 78°C until no more liquid is collected in the beaker. (Note that very little water distills over because 78°C is less that the boiling point of water. Almost all the water vapour will condense before reaching the top of the fractionating and falls back to the distillation flask).
3. Replace the beaker with another beaker.
4. Now, heat the mixture until the temperature at the top of the fractionating column is 100°C (which is the boiling point of water). The water vapour passes into the condenser, gets cooled and collects in a beaker kept at the other end of the condenser.

Questions

1. What changes of state are taking place in the fractionating column?
2. Explain why we can use fractional distillation to separate ethanol and water.

• Show Solutions
  1. a) evaporation b) condensation
  2. We can use fractional distillation because the boiling points of ethanol (78°C) and water (100°C) are different.

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