91�ȱ�

Learn more on distillation and chromatography in this podcast.

Listen to the full series on 91�ȱ� Sounds.

Simple distillation

Simple distillation is used to separate a solvent from a solution. It is useful for producing water from salt solution.

Simple distillation works because the dissolved solute has a much higher boiling point than the solvent. When the solution is heated, solvent vapour evaporates from the solution. The gas moves away and is cooled and condensed. The remaining solution becomes more concentrated in solute as the amount of solvent in it decreases.

Fractional distillation

Fractional distillation is used to separate different liquids from a mixture of liquids. It is useful for separating ethanol from a mixture of ethanol and water, and for separating different fractions from crude oil.

Fractional distillation works because the different liquids have different boiling points. When the mixture is heated:

• vapours rise through a column which is hot at the bottom, and cold at the top
• vapours condense when they reach a part of the column that is below the temperature of their boiling point
• the liquid flows out of the column

There are two ways of obtaining different liquids from the column:

• By collecting different liquids from different parts of the column. The substance with the lowest boiling point is collected at the top of the column.
• By continuing to heat the mixture to increase the temperatures in the column. The substance with the lowest boiling point is collected first.

Different liquids can be obtained from different parts of the column, or by continuing to heat the mixture to increase the temperatures in the column.