What you'll learn
Most substances around us are mixtures, not pure elements or compounds, and chemists need ways to separate them. For AQA GCSE Chemistry you need to understand what a mixture is, the difference between a pure substance and a mixture, and how to choose the right separation technique — filtration, crystallisation, evaporation, distillation and chromatography. This guide covers the meaning of purity, each separation method and when to use it, and how the technique depends on the properties of the substances. By the end you should be able to define a mixture, choose the correct separation technique for a given mixture, and explain how each one works.
Key terms and definitions
Pure substance — A single element or compound, not mixed with anything else.
Mixture — Two or more substances mixed together but not chemically combined.
Filtration — Separating an insoluble solid from a liquid using filter paper.
Crystallisation — Forming crystals of a dissolved solid by evaporating the solvent.
Distillation — Separating a liquid from a mixture by evaporating and then condensing it.
Chromatography — Separating substances in a mixture based on how they move through a medium.
Solvent — The liquid in which a solute dissolves.
Solute — The substance that dissolves in a solvent to form a solution.
Core concepts
Pure substances and mixtures
In chemistry, a pure substance is a single element or compound with nothing else mixed in. A mixture contains two or more substances that are not chemically joined, so they keep their own properties and can be separated by physical means. Because the substances in a mixture are not chemically bonded, separating them does not require a chemical reaction — only a physical process such as filtering or evaporating.
A useful test of purity is melting or boiling point: a pure substance melts or boils at a single, specific temperature, while a mixture melts or boils over a range of temperatures.
Filtration
Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel. The liquid (the filtrate) passes through the tiny holes in the paper, while the solid particles are too big to pass and are left behind as the residue. For example, filtration separates sand from water. Filtration works because one substance is solid and insoluble while the other is a liquid.
Crystallisation and evaporation
Crystallisation separates a soluble solid from a solution. The solution is heated to evaporate some of the solvent, then left to cool. As it cools, the dissolved solid comes out of solution as crystals. This method is used to obtain a salt from its solution, and gives good crystals if done slowly. Simple evaporation removes all the solvent by heating until only the solid is left, but this can decompose some substances and does not give such good crystals, so crystallisation is preferred where the shape of the crystals matters.
Distillation
Distillation separates a liquid from a solution, or separates two liquids with different boiling points. There are two types:
- Simple distillation separates a solvent from a solution — for example, obtaining pure water from salty water. The solution is heated, the water evaporates, and the vapour is cooled in a condenser and collected as pure liquid, leaving the salt behind.
- Fractional distillation separates a mixture of two or more liquids with different boiling points, such as different liquids in crude oil or a mixture of ethanol and water. The liquid with the lower boiling point evaporates first and is collected, then the temperature is raised to collect the next.
Distillation relies on the substances having different boiling points.
Chromatography
Chromatography separates substances in a mixture based on how well they dissolve in a solvent and how they move through a medium such as paper. A spot of the mixture is placed on chromatography paper, and the bottom is dipped in a solvent. As the solvent moves up the paper, it carries the substances with it. Substances that are more soluble and less attracted to the paper travel further; those less soluble travel less far, so the components separate into spots. This is used to separate coloured substances such as inks and food colourings, and to check the purity of a substance — a pure substance produces only one spot.
Choosing the right technique
The correct method depends on the properties of the substances in the mixture:
| Mixture | Technique |
|---|---|
| Insoluble solid + liquid | Filtration |
| Soluble solid from its solution | Crystallisation |
| Solvent from a solution | Simple distillation |
| Two liquids with different boiling points | Fractional distillation |
| Coloured substances / checking purity | Chromatography |
How paper chromatography is measured: the Rf value
Chromatography results can be measured using the Rf value, which describes how far a substance has travelled compared with the solvent. It is calculated as:
Rf = distance moved by the substance ÷ distance moved by the solvent
The Rf value is always between 0 and 1, because the substance cannot travel further than the solvent. Each substance has its own Rf value in a particular solvent, so Rf values can be used to identify unknown substances by comparing them with known ones. A substance that is very soluble and weakly attracted to the paper has a high Rf value, while one that is less soluble has a low Rf value. Being able to calculate an Rf value from a chromatogram is a common exam skill.
Potable water as an application
Separation techniques matter in everyday life, for example in producing potable (drinkable) water. Fresh water is treated by filtering it to remove solids and then sterilising it to kill microbes. Where fresh water is scarce, sea water can be made drinkable by distillation, which separates the pure water from the dissolved salts, though this uses a lot of energy. This shows how the separation methods studied in this topic — filtration and distillation — are applied on a large scale to solve a real problem.
Worked examples
Example 1: Choosing a method
Which technique would you use to separate sand from a mixture of sand and water? Filtration, because sand is an insoluble solid and water is a liquid. The sand stays on the filter paper while the water passes through.
Example 2: Getting salt and water separately
You have salty water and want to obtain both pure water and the salt. Describe how. Use simple distillation: heat the solution so the water evaporates, then cool the vapour in a condenser to collect pure water. The salt is left behind in the flask, so both are obtained.
Example 3: Testing purity with chromatography
Explain how chromatography can show whether a food colouring is a single pure substance or a mixture. A spot of the colouring is placed on chromatography paper and a solvent allowed to move up. If the colouring is pure, it produces a single spot. If it is a mixture, it separates into several spots, one for each substance.
Example 4: Recognising a pure substance
How can a melting point show that a substance is pure? A pure substance melts at a single, specific temperature. If the substance melts over a range of temperatures instead, it is a mixture, not pure.
Common mistakes and how to avoid them
A common error is confusing filtration and crystallisation. Filtration removes an insoluble solid; crystallisation recovers a soluble solid from its solution. Check whether the solid dissolves before choosing.
Students often mix up simple and fractional distillation. Simple distillation separates a solvent from a solution; fractional distillation separates two or more liquids with different boiling points. Name the right one.
Another mistake is thinking separation techniques involve chemical reactions. Because a mixture is not chemically bonded, separation uses physical processes only, based on differences in properties.
When explaining chromatography, do not just say "the colours separate". Explain that more soluble substances travel further up the paper, so the components spread out into different spots.
Finally, remember that a pure substance has a single, sharp melting or boiling point, while a mixture melts or boils over a range — this is a common way to test purity.
Exam technique for "Mixtures and separation techniques"
The most common question type gives you a mixture and asks which technique separates it and why. Match the method to the properties: insoluble solid → filtration, soluble solid → crystallisation, solvent from solution → simple distillation, two liquids → fractional distillation, coloured substances → chromatography.
For "describe how" questions, give the steps in order and name the equipment (filter paper and funnel, condenser, chromatography paper). Explaining why the method works — different boiling points, or one substance being insoluble — earns the higher marks.
Be ready to use melting and boiling points to identify a pure substance, and to explain chromatography in terms of solubility and movement up the paper. Use precise terms — filtrate, residue, solvent, solute — throughout.
Quick revision summary
- A pure substance is a single element or compound; a mixture is substances mixed but not chemically joined, separable by physical means.
- Filtration: insoluble solid from a liquid.
- Crystallisation: soluble solid recovered from its solution by evaporating solvent and cooling.
- Simple distillation: solvent from a solution; fractional distillation: liquids with different boiling points.
- Chromatography: separates substances by solubility and movement up paper; a pure substance gives one spot.
- A pure substance has a single melting/boiling point; a mixture melts or boils over a range.