Filtration and crystallisation

Filtration

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 reactant from a reaction mixture.

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

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

Separating insoluble solids

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

Crystallisation

Crystallisation is used to produce solid crystals from a solution. When the solution is warmed, some of 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 from crystallisation:

  • put the solution in an evaporating basin
  • warm the solution by placing the evaporating basin over a boiling water bath
  • stop heating when crystals begin to form around the edge of the basin

After the remaining solution has cooled down, pour the excess liquid away (or filter it). Dry the crystals using a warm oven or by patting them with filter paper.