Filtration and crystallisation

Separation techniques

The production of a chemical does not necessarily produce a pure sample of the chemical. It may contain impurities or start as a mixture of substances. Separation techniques are used to separate the useful product from any impurities or by-products.

Separation techniques work by using differences in the physical properties of substances in the mixture:

Separation processes are not always completely successful the first time. Repeated processes are sometimes needed to achieve acceptable purity.


Filtration is used to separate an insoluble solid from a liquid. It is useful for separating sand from a mixture of sand and water, or excess solid reactant from a solution.

Filtration works because the filter paper has tiny holes or pores in it. These are large enough to let simple molecules and dissolved ions through, but not the much larger particles of undissolved solid.

The solid and liquid mixture is poured into the filter funnel.

1. One beaker contains a mixture of solid and liquid, the other contains a funnel with filter paper


Crystallisation is used to produce solid crystals from a solution. When the solution is warmed, the solvent evaporates leaving crystals behind. For example, crystallisation is used to obtain copper sulfate crystals from copper sulfate solution.

A solution is placed in an evaporating basin and heated with a Bunsen burner

1. A solution is placed in an evaporating basin and heated with a Bunsen burner

To obtain large, regularly shaped crystals:

  • put the solution in an evaporating basin
  • warm the solution by placing the evaporating basin over a boiling water bath
  • stop heating before all the solvent has evaporated

The evaporating basin may then be left, allowing the crystals to grow.