Evaporation and Crystallisation

To understand crystallisation and evaporation, it is important to understand what a solvent, solute and solution are.

  • Solute – Substance that dissolves to form a solution
  • Solvent – A substance that dissolves a solute to form a solution
  • Solution – Formed when a solute dissolves in a solvent


Evaporation is the process by which a liquid changes into a gas. This process occurs when molecules of a liquid are given enough energy to escape the surface of the liquid and become a gas. This energy is usually provided by heat.

  • Evaporation is an important process in the formation of crystals

As evaporation changes a liquid to a gas below its boiling point, it is not the same as boiling.

Separating a solution

Evaporation can be used to separate a solid solute from a solvent in a solution. When the solution is heated, the solvent (the component present in large amounts) evaporates faster than the solute (the component present in smaller amounts).

This process is used to obtain a solid substance from a solution. Once the solvent has evaporated, the solute is left behind as a solid.

In the lab, the solution is typically heated by a Bunsen burner in an evaporating basin.

An example of this is when a saltwater solution is heated. The water will evaporate more quickly than the salt, which leaves behind a more concentrated solution of salt. This process can be used to purify the salt or to obtain a specific concentration of salt in the solution.

Evaporation is a useful technique for separating mixtures because it is relatively simple and does not require special equipment.


Crystallisation is a separation technique that is used to separate a solvent from a solution, producing solid crystals. When the solution is heated, some of the solvent evaporates, which leaves behind a more concentrated solution.

During crystallisation, the solvent is evaporated, leaving the solute behind. However, crystallisation is different to the evaporation method because it takes place over a longer period of time and it produces larger crystals.

Below are the steps for the crystallisation process.

1. Gently heat the solution in an evaporating basin to evaporate some of the solvent and leave a more concentrated solution behind.

  • Carefully monitor the temperature, and avoid overheating which may cause chemical breakdown

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

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

4. As the solution cools, 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, and collect them in a dry container.

6. Finally, dry the crystals thoroughly by leaving them in a warm place, such as an oven, to remove any remaining solvent or water