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Fractional distillation

Fractional distillation

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True or false? Simple distillation can be used to separate the ingredients of a homogenous mixture if they have very different boiling points.

Fractional distillation

Many homogenous mixtures, for example saltwater, can be separated by evaporating and then condensing one of the ingredients. This method is called simple distillation. It works very well if you want to separate a liquid from a solid, or if the ingredients in the mixture evaporate and condense at very different temperatures — if they have very different boiling points. But how about this mixture of water and ethanol? Both substances in this mixture are liquid, and the difference between their boiling points is less than 25°C.

Simple distillation won’t work here. To separate such a mixture, we need a different type of distillation — fractional distillation. Just like simple distillation, fractional distillation is based on evaporation and condensation. But it uses a special set-up which allows for greater precision. Let’s take a look at a typical fractional distillation set-up.

The mixture to be separated — in our case, water mixed with ethanol — is contained in this round-bottom flask. The flask is placed above a source of heat, which will be used to raise the temperature of the mixture inside. The top of the flask is attached to a long vertical glass tube, called a fractionating column. The fractionating column is long enough to create a temperature gradient - hotter at the bottom, and colder at the top. The fractionating column also contains parts that increase its surface area, for example glass beads or a series of trays.

Ethanol has a lower boiling point and is the more volatile of the mixture’s two ingredients. So, when you heat the mixture, ethanol starts turning into a vapour faster than water. The ethanol vapour rises up the fractionating column, which is still cold. The ethanol vapour releases heat into the column, and the vapour’s temperature drops below its boiling point. This causes the ethanol to condense.

Some of the liquid drips back into the flask, and some stays condensed on the inside surface of the fractionating column. Let’s keep adding heat. The temperature of the ethanol rises again, above its boiling point. This time, the fractionating column is a bit warmer than it was before. The ethanol vapour can rise a bit higher than the first time, before it condenses again.

This process of heating up, evaporating, rising up the column, cooling, and condensing happens several times; until the vapour finally reaches the top of the fractionating column. At the top of the fractionating column, there is an adapter, which links the fractionating column with the next parts of the set-up. The adapter is usually fitted with a thermometer, so you can measure the temperature at the top of the fractionating column. When the vapour reaches the top of the fractionating column, it is directed through the adapter to a long double-walled tube, called a condenser. The space between the inner and outer wall of the condenser is filled with cold water.

This cools the ethanol and brings its temperature below its boiling point, so it condenses. The liquid ethanol drips down through the condenser and collects in an empty flask or beaker placed below. And what happens to the water? As the temperature of the mixture in the flask rises, at some point the water reaches its boiling point too. But due to the temperature gradient inside the fractionating column, the water vapour doesn’t go up the column as fast as the ethanol vapour.

In the end, all the ethanol from our mixture evaporates and moves to the receiver, while the water stays in the first flask. Thanks to fractional distillation, the ingredients of the mixture have been separated!