Choosing purification techniques

The method used to separate a mixture will depend on the properties of the substances in the mixture.

Insoluble solid and a liquid

The solid is removed from the liquid using filtration. The filter paper traps the particles of insoluble solid but lets the liquid through.

Soluble solid from a solution

The solution is warmed to evaporate the solvent. Once the solution is concentrated, it is left to cool. This allows the soluble solid to form by crystallisation.

Solvent from a solution

The solvent is removed using simple distillation. As the solvent is distilled off, the dissolved solid remains behind.

A mixture of liquids

The liquids are heated and, because they have different boiling points, they will evaporate at different temperatures. This process is fractional distillation.

A mixture of soluble solids from a solution

Paper chromatography is used to separate mixtures of soluble substances. These are often coloured substances such as food colourings, inks, dyes or plant pigments.

Examples of purification techniques

MixtureTechniqueReason
Ethanol and waterFractional distillationEthanol is a liquid which dissolves in water
Water from sea waterSimple distillationWater is a solvent for a number of dissolved solids in sea water
Copper(II) sulfate from copper(II) sulfate solutionCrystallisationCopper(II) sulfate is a dissolved solid
Chlorophyll from plant leavesChromatographyChlorophyll is one of a number of dissolved pigments in plant leaves
Sand from sea waterFiltrationSand is an insoluble solid in sea water