3.7 - Filtration & Crystallisation

Filtration

• Filtration separates an insoluble solid from a liquid.
• Filter paper has tiny holes that let liquid particles through, but trap solid particles.
• The paper is placed in a filter funnel and the mixture is poured in:
  • Residue - The solid left behind on the filter paper.
  • Filtrate - The liquid that passes through the filter paper.
• Filtration can't separate a dissolved solid from a liquid, because dissolved particles are small enough to pass straight through the filter paper.

Evaporation

• Evaporation separates a soluble solid from a liquid by heating the solution until all the liquid has evaporated, leaving dry crystals behind.
• The solution is placed in an evaporating dish and heated. As the solvent evaporates, the solution becomes more concentrated until crystals form.

Limits of evaporation

• Some solids decompose (break down) when heated strongly, so they shouldn't be evaporated to dryness.
• The liquid evaporates into the air and isn't collected.
• It produces tiny crystals rather than large, well-formed ones.

Crystallisation

Crystallisation separates a dissolved solid from a liquid without heating to dryness, producing larger and purer crystals.

Method

1. Gently heat the solution in an evaporating dish until some of the liquid evaporates and crystals start to form (crystallisation point).
2. Remove from the heat and leave the solution to cool slowly.
3. As it cools, the solid becomes less soluble and more crystals form.
4. Filter the crystals from the remaining liquid.
5. Dry the crystals (e.g. in a warm oven, with filter paper, or in a desiccator).

Why use crystallisation

• It prevents solids from decomposing under strong heat.
• It produces purer crystals because impurities remain dissolved in the solution.