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 solution.

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 the solvent evaporates leaving behind a more concentrated 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

After the remaining solution has cooled down, pour the excess liquid away (or filter it). Dry the crystals using a warm oven or in air.