Chemistry, Third edition

19 · SEPARATING MIXTURES

Miscible liquids

Two miscible liquids can be separated by simple distillation (Fig. 19.7) if one liquid

boils at least 25 °C higher than the first. If the difference between their boiling points

is less than 25 °C, then miscible liquids can be separated by fractional distillation.

The apparatus is similar to that for ordinary distillation, except that a fractionating

columnis inserted between the flask and the condenser (Fig. 19.9). A fractionating

column is a glass column packed with glass beads, or some other unreactive material

that has a high surface area, upon which vapour can condense.

The less volatile a liquid is, the higher is its boiling point. When the mixture of

liquids is heated, the vapour of both liquids passes up the column but the vapour of

the liquid with the higher boiling point(water in this example) will tend to condense

in the column and run back down into the flask. The vapour of the lower boiling point

liquidtends to enter the condenser, condense and pass into the receiving beaker. If

the boiling points of the two liquids are not too close, good separations can be

achieved. In the example chosen (ethanol and water), a liquid much richer in

ethanol (the distillate) is collected in the receiving flask, because ethanol (boiling

point 78 °C) is the more volatile component.

In distillation, the separation depends on equilibria being established between

descending liquid and rising vapour and is best achieved by heating the mixture as

slowly as possible. Fractional distillation can be used to separate more than two

components. It is used in industry to separate crude oil into several fractions. Each

fraction contains components with boiling points that are close together.

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Fig. 19.9Fractional distillation.