2.5 - 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.

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.

Purifying rock salt

• Rock salt is a mixture of salt (soluble in water) and sand (insoluble).
• Differences in solubility allow the two solids to be separated using filtration and crystallisation.

The four steps

1. Grind the rock salt with a pestle and mortar to increase its surface area, so the salt dissolves faster.
2. Dissolve the mixture in water and stir so the salt dissolves while the sand remains insoluble.
3. Filter the mixture through filter paper in a funnel. The sand stays behind as the residue; the salt solution passes through as the filtrate.
4. Crystallise the salt from the filtrate by heating it gently, allowing it to cool, and drying the crystals.