Separation techniques

Separation techniques are physical methods for separating the substances in a mixture. For GCSE Chemistry there are five: filtration, crystallisation, simple distillation, fractional distillation and chromatography. Which one you use depends on what is in the mixture: a solid and a liquid, a dissolved solid, or two liquids.

Mixtures and separation

AQA's GCSE specification defines a mixture as two or more elements or compounds not chemically combined together. Because the substances aren't bonded to each other, they can be separated by physical processes, which AQA notes do not involve chemical reactions: no new substances are made. You need to be able to describe and explain each process and suggest a suitable method for a given mixture.

Which separation technique should I use?

Filtration

Pour the mixture through filter paper in a funnel. The liquid, called the filtrate, passes through; the insoluble solid, the residue, stays on the paper. Use it to remove an insoluble solid, such as excess metal oxide left over after making a salt, or sand from water.

Crystallisation

1. Warm the solution gently in an evaporating basin so that part of the solvent evaporates.
2. Stop when crystals start to form at the edge, or a drop on a glass rod forms crystals as it cools.
3. Leave the solution to cool, so more crystals form.
4. Filter off the crystals and dry them, for example in a warm oven or between filter papers.

Crystallisation keeps the solid. It is better than evaporating to dryness for salts that break down when heated strongly, and it gives larger, purer crystals.

Simple distillation

Simple distillation separates a liquid from a solution, keeping the liquid. Heat the solution in a flask. The solvent boils, the vapour passes into a condenser, where cold water cools it back to a liquid, and the pure liquid, called the distillate, drips into a beaker. The dissolved solid stays in the flask. Distilling salty water gives pure water.

Fractional distillation

Fractional distillation separates a mixture of liquids with different boiling points. A fractionating column sits between the flask and the condenser. It is hottest at the bottom and coolest at the top, so vapours condense and re-evaporate many times as they rise. The liquid with the lowest boiling point reaches the top first and passes into the condenser; liquids with higher boiling points follow as the temperature rises.

A mixture of ethanol and water can be separated this way, because ethanol boils at a lower temperature than water. On a huge scale, fractional distillation separates crude oil into fractions such as petrol and diesel.

Chromatography

Paper chromatography separates a mixture of dissolved substances, such as the dyes in an ink. Put a small spot of the mixture on a pencil start line drawn a little way up the paper, then stand the paper in a solvent below that line. As the solvent moves up the paper it carries each substance a different distance. A pure substance gives one spot; a mixture gives more than one. Each spot's R_f value (distance moved by the spot divided by distance moved by the solvent) helps identify it. The thin-layer chromatography guide covers the A-level version.

Separating a mixture in several steps

Some mixtures need more than one technique. To get pure, dry salt from a mixture of salt and sand:

1. Add water and stir: the salt dissolves, the sand doesn't.
2. Filter: the sand stays on the filter paper; the salt solution passes through.
3. Crystallise the salt solution to get salt crystals.
4. Rinse the sand with water and dry it if you want to keep it too.

Common mistakes

• Using filtration for a dissolved solid. Dissolved substances pass straight through filter paper.
• Confusing simple and fractional distillation. Simple distillation separates a liquid from a solid in solution; fractional distillation separates liquids from each other.
• Evaporating to dryness when crystallising. Stop when crystals start to appear and let the rest form as it cools.
• Saying separation makes new substances. These are physical processes; no chemical reactions happen.
• Drawing the chromatography start line in ink. Use pencil, or the ink will separate too.

To practise separation and purification questions in a short timed paper, try today's GCSE Chemistry Higher or GCSE Chemistry Foundation paper.

Read next

Chemistry · AnalysisThin-layer chromatography (TLC): method and Rf valuesHow thin-layer chromatography (TLC) works: the method, Rf values, why compounds separate, and how it compares with column and gas chromatography.