Chemguide: Core Chemistry 14 - 16

Fractional distillation

Fractional distillation is what you would use, for example, to get the alcohol produced by the fermentation of sugar. It lets you produce almost pure alcohol from a mixture which contains yeast, sugar and lots of water.

It is used to separate mixtures of liquids into the individual liquids, and relies on the difference between their boiling points.

Fractionally distilling a water / alcohol mixture

I am using this example because it is the most likely case of fractional distillation you will come across in the lab at this level.

You will find later that fractional distillation is also done on a large scale, for example in the oil industry. The principle is the same, but the details are quite different.

This video (from the same source as the one you may already have looked at about simple distillation) gives as good an explanation of fractional distillation as you will find at this level.

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

In the flask both of these turn to vapour.

As the column heats up due to the rising hot vapours, the temperature on the thermometer will eventually hit 78°C. At that point, the vapour at the top of the column will be almost all alcohol. The vapour escapes into the condenser, turns back to a liquid and can be collected.

The skill now is to adjust the heating so that the temperature stays at 78°C. If you let it go above that, the vapours around the thermometer will contain a mixture of water and alcohol - you don't want that!

When you do this in the lab, you aren't going to be interested in collecting the water and so as soon as the temperature starts to go up above 78°C, you just stop heating.

For theoretical reasons impossible to explain at this level, you can't actually get 100% alcohol from fractional distillation - the best you can do is 96%.

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