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The set-up for simple distillation and fractional distillation in terms of the bulb of the thermometer, condenser, and boiling chips is the same, and the physical property of the substances that both types of distillation is centred on is the difference in boiling point
However, there are two main differences between the set-up:
Type of mixture
Simple distillation: Contains a mixture of soluble solid and liquid (solution) (e.g., salt and water)
Fractional distillation: Contains a mixture of miscible liquids (e.g., ethanol and water - liquids that can mix)
Presence of the fractionating column
Simple distillation: Uses a distillation flask that is directly attached to the condenser.
Fractional distillation: Uses a round-bottomed flask and a fractionating column (containing glass beads)
The purpose of the fractionating column is related to the problem that arises when trying to separate a mixture of two miscible liquids with the simple distillation set-up.
What problems arise when the simple distillation set-up is used to separate two miscible liquids?
The boiling points of two miscible liquids are usually very close to one another
E.g., the boiling point of ethanol is 78°C, while the boiling point of water is 100°C
As a result, this can affect the purity of the distillate
The temperature after all the ethanol has distilled over will rise very quickly, meaning that water can start distilling over and affect the purity of the ethanol if the conical flask containing the distillate is not changed in time.
Evaporation can occur, which means that some water vapour can still enter the condenser and be condensed together with ethanol at 78°C
Purpose of the fractionating column
The fractionating column helps to increase surface area for condensation of vapour of the liquid with the higher boiling point back into the round-bottom flask.
How does this help the problem?
Glass is a material that is very good at providing a cool surface area for hot vapour to condense on (e.g., why the bathroom mirrors always fogs up in a warm shower)
At the boiling point of the lower boiling point liquid, it is very easy for any vapour of the higher boiling liquid to condense due to the lack of thermal energy needed to overcome the stronger attractive forces of the particles of the higher boiling point liquid (temperature not at their boiling point)
The glass beads in the fractionating column helps to delay the time it takes for the vapour of the higher boiling liquid to reach the condenser, therefore allowing for all of the lower boiling point liquid to be collected as distillate first
The fractionating column is also tall, which means that the temperature at the top cooler than the bottom.
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