1.4 Filtration and crystallisation

Choosing a separation method

Because the parts of a mixture are not chemically joined, they can be separated by physical processes. The method depends on their properties.

Filtration

Used to separate an insoluble solid from a liquid, such as sand from water, or excess copper oxide from copper sulfate solution.

1. Fold a filter paper into a cone and put it in a filter funnel over a beaker or conical flask.
2. Pour the mixture in. The liquid passes through the tiny holes in the paper as the filtrate.
3. The solid particles are too big to pass through, so they stay as the residue.

Crystallisation

Used to get a soluble solid from a solution, such as salt from salt water or copper sulfate crystals.

1. Pour the solution into an evaporating basin.
2. Heat it gently, ideally over a water bath, to evaporate some of the water until crystals start to form at the edge (the solution is saturated).
3. Leave it to cool: crystals form as the solubility falls.
4. Filter off the crystals and dry them between filter papers or in a warm oven.

Heating gently, rather than boiling the solution dry, gives larger, purer crystals and does not decompose solids such as hydrated salts.

Example: rock salt

Rock salt is salt mixed with sand. Add water and stir to dissolve the salt, filter to remove the sand, then crystallise the salt from the filtrate.