A mixture contains two or more substances that are not chemically bonded. Because of this, mixtures can be separated using physical methods. These methods do not involve chemical reactions and rely on differences in properties like size, boiling point, or solubility.

Key definitions

• Soluble – a substance that can dissolve in a liquid (e.g. salt is soluble in water)
• Insoluble – a substance that cannot dissolve in a liquid (e.g. sand is insoluble in water)
• Solvent – the liquid in which a substance is dissolved (e.g. water is the solvent in saltwater)

Filtration

Filtration is used to separate an insoluble solid from a liquid.

Example:

Separating sand (insoluble) from saltwater (solution).

Method:

1. Pour the mixture through filter paper in a funnel.
2. The solid (residue) stays on the paper.
3. The liquid (filtrate) passes through.

Filtration is useful in both chemistry and everyday life – for example, in coffee machines or water purification.

Crystallisation

Crystallisation separates a soluble solid from a solution by forming crystals.

Example:

Getting salt crystals from saltwater.

Method:

1. Gently heat the solution to evaporate some water.
2. Leave it to cool slowly.
3. Crystals form as the solution becomes saturated.
4. Filter and dry the crystals.

Crystallisation is slower than simple evaporation but gives larger, purer crystals.

Chromatography

Chromatography separates substances in a mixture of dyes or inks based on how well they dissolve and move through paper.

Example:

Separating colours in black ink.

Method:

1. Draw a line near the bottom of chromatography paper.
2. Place a spot of ink on the line.
3. Dip the paper into a solvent (e.g. water), but keep the line above the surface.
4. The solvent moves up the paper, carrying the ink with it.
5. Different dyes travel different distances.

💡 Each dye moves at a different speed depending on its solubility and attraction to the paper.

Distillation

Distillation separates a liquid from a solution based on boiling points.

Simple distillation:

Used to separate a liquid from a dissolved solid (e.g. pure water from salty water).

Method:

1. Heat the solution.
2. The liquid evaporates and enters a condenser.
3. The vapour cools and condenses back to a liquid.
4. The solid is left behind.

Fractional distillation:

Used to separate two or more liquids with different boiling points (e.g. ethanol and water).