Filtration and Crystallisation


Filtration is a separation technique used to separate insoluble solids from a liquid. For example, separating sand from a mixture of sand and water.

Filter paper is placed in a funnel above a beaker or flask. The mixture of a liquid and an insoluble solid is then poured into the funnel. Since the filter paper contains many tiny holes, liquid particles and dissolved ions can pass through. However, the larger insoluble solid particles will be unable, so they will build up as residue in the filter paper.

Diagram showing a mixture of liquid with insoluble solid being poured from a glass beaker into a filter funnel. The filtrate is collected in the conical flask below.

  • You can use a glass rod to allow smaller particles to pass through the filter paper.


Crystallisation is a separation technique used to separate a soluble solid from a liquid. The process works by forming solid crystals from a saturated solution.

First, it is important to understand what a solvent, solute, and solution are.

The solvent is the component of the solution that is present in the largest amount and is typically a liquid. The solute is the component that is dissolved in the solvent and is present in a smaller amount. When these two components are combined, they form a solution.

During crystallization, the solution is allowed to cool and/or have its concentration increased, causing the solute to become saturated and begin forming solid crystals. These crystals can then be separated from the liquid by filtration or centrifugation, leaving behind a purified solid and a concentrated solution.

Diagram depicting a solute and solvent being combined to form a solution.
  • Solute – Substance that dissolves in the solvent to form a solution
  • Solvent – The substance in which the solute dissolves to form a solution
  • Solution – Formed when a solute dissolves in a solvent

So, a saturated solution is a solution that contains the maximum amount of solute that can be dissolved in the solvent. Any more solute added to the solution will not dissolve and will remain in a solid state.


The process of filtration involves the following steps:

1. The solution is gently heated in an evaporating basin, which will evaporate some of the solvent, leaving behind a more concentrated solution.

2. As more of the solvent evaporates, you will see crystals start to form along the edge of the basin.

Diagram illustrating a solution heated by a Bunsen burner. An arrow highlights the evaporation of water and the crystallised solute collecting at the basin's bottom.

3. Stop heating the solution before all of the solvent evaporates, then leave the solution to cool

  • Some chemicals will break down when we heat them, so ensure that the heating process will not affect the chemical that you aim to crystallise. It may be better to heat the solution in a water bath or leave it to evaporate on its own.

4. As the solution begins to cool, more crystals will start to form, because solutes are more concentrated in cooler temperatures.

5. Filter out the crystals from the remaining solution using filter paper and a funnel.

6. Leave the crystals somewhere warm so they can dry.

  • For example, in an oven