Filtration and crystallisation

Learn more on filtration, evaporation and crystallisation in this podcast.

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Filtration

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

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

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Separating insoluble solids
1. One beaker contains a mixture of solid and liquid, the other contains a funnel with filter paper
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2. The solid and liquid mixture is poured into the filter funnel
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3. The liquid drips through the filter paper but the solid particles are caught in the filter paper

Crystallisation

is used to produce solid from a solution. When the solution is warmed, some of the leaving behind a more concentrated solution.

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1. A solution is placed in an evaporating basin and heated with a Bunsen burner.
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2. The volume of the solution has decreased because some of the water has evaporated. Solid particles begin to form in the basin.
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3. All the water has evaporated, leaving solid crystals behind.

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.

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Separation and purification - EdexcelFiltration and crystallisation