Filtration and crystallisation

In a nutshell

Separation techniques are important to scientists for increasing the purity of samples. This summary looks at the techniques filtration and crystallisation and their purposes, experimental set ups and common examples.

Filtration

Definition

Filtration is a way of separating insoluble salts from solutions. The main purpose of filtration is to purify a solution by removing the solids.

Note: A solution is a solute dissolved in a solvent. The mass of solute that dissolves in a given volume of solvent at a given temperature is the solubility.

Example

Removing debris from sea water.

Filtration also can be used to obtain a solid by removing the solution.

Example

Obtaining crystals.

Chemistry; States of matter and mixtures; KS4 Year 10; Filtration and crystallisation — Filtration apparatus. 1. Funnel, 2. Filter, 3. Mixture, 4. Residue, 5. Filtrate.

Procedure

1.	Grind the substance to make dissolving easier.
2.	Mix with water and stir.
3.	Filter. The solute will not pass through but the solvent will.
4.	Allow excess water to evaporate off from the solute.

How does filtration work?

The filter paper allows solutions to pass through but not solids, acting as a separation technique.

Example

Separation of rock salt (a mixture of salt and sand). Salt is soluble in water but sand is not. Because of this, they can be separated. The sand will not pass through the filter paper but the salt will as it is dissolved in the water. The salt can be separated from the water through evaporation or crystallisation.

Crystallisation

Definition

Crystallisation is a way of separating soluble salts from solutions. The main purpose of crystallisation is to obtain solid crystals from a solution.

Chemistry; States of matter and mixtures; KS4 Year 10; Filtration and crystallisation — Crystallisation apparatus. 1. Solution, 2. Hot water.

Procedure

1.	Add the solution to the evaporating basin and heat. This removes some water making the solution saturated (cannot hold any more solute).
2.	Allow the solution to cool.
3.	Crystals will precipitate out.
4.	Dry the crystals and collect.

How does crystallisation work?

The solubility of the solute decreases as the saturated solution cools, triggering the formation of crystals from the excess solute.

Example

Separation of copper sulfate.

Evaporation is an alternative technique that can also be used to separate soluble salts from solution. It is much quicker than crystallisation, however you can only use it if the salt does not decompose (break down) by heat.