Methods of Separating & Purifying SubstancesEdexcel GCSE Chemistry: Revision notes
Section 1
What does 'pure' mean in chemistry, and how is it different from everyday use?
In everyday language, 'pure' often means something is natural or unadulterated (e.g. 'pure orange juice'). In chemistry, a pure substance is a single element or compound, not mixed with any other substance — it contains only one type of particle throughout.
A mixture is two or more elements or compounds not chemically combined, which can be separated by physical methods.
A pure substance melts and boils at one exact, specific temperature. A mixture melts and boils over a range of temperatures.
Do not confuse the everyday meaning of 'pure' (natural/unprocessed) with the chemistry meaning (a single substance, chemically) — exam answers must use the chemistry definition.
Section 2
How does melting point data distinguish pure substances from mixtures?
Melting point data can identify purity:
- A pure substance has a sharp, exact melting point — it melts entirely at one specific temperature
- A mixture melts over a range of temperatures, and usually melts at a lower temperature than the pure substance would
If you are given melting point data for an unknown sample and asked whether it is pure, always compare the range: a narrow, single-temperature melt indicates purity; a broad range indicates a mixture (or impure sample).
When interpreting melting point data, state both features together: 'the substance melted over a range of temperatures, so it is a mixture/impure' — a single feature without the conclusion loses marks.
Section 3
What separation technique suits which type of mixture?
| Technique | Separates | How it works |
|---|---|---|
| Filtration | An insoluble solid from a liquid | The liquid passes through filter paper; the solid residue is left behind |
| Crystallisation | A soluble solid (solute) from a solution | The solution is heated to evaporate solvent until saturated, then cooled so pure solid crystals form |
| Simple distillation | A solvent from a solution (e.g. water from salt solution) | The mixture is heated; the solvent evaporates, is cooled/condensed, and collected separately, leaving the solute behind |
| Fractional distillation | Two or more miscible liquids with different boiling points | The mixture is heated; each liquid evaporates and is collected separately at its own boiling point, using a fractionating column |
| Paper chromatography | Soluble substances within a mixture (e.g. dyes/inks) | Different substances move at different rates through the stationary phase (paper), carried by the moving solvent (mobile phase) |
The correct technique is chosen based on the known properties of the mixture's components (e.g. solubility, boiling point, particle size).
To separate the solvent from ink and see its individual coloured dyes, simple distillation and paper chromatography are used together — this is the core practical for this subtopic.
Section 4
How does paper chromatography work, and how are results interpreted?
In paper chromatography, a spot of the mixture is placed near the bottom of a piece of paper. The paper is placed in a solvent (the mobile phase), which moves up through the paper (the stationary phase), carrying the substances in the mixture with it. Substances that are more soluble in the solvent, or interact less strongly with the paper, move further and faster; different substances move at different rates, so they separate into distinct spots.
Interpreting a chromatogram:
- A pure substance produces only one spot; an impure substance/mixture produces multiple spots
- Substances can be identified by comparing spots to known reference substances run on the same chromatogram — matching position (and colour) suggests the same substance
- Substances can also be identified by calculating the Rf value: Rf = distance travelled by substance ÷ distance travelled by solvent. Comparing calculated Rf values to known reference Rf values identifies the substance.
Always give the Rf formula exactly as 'distance moved by substance divided by distance moved by solvent' when asked to define or calculate it.
Section 5
How is water made potable?
Potable water is water that is safe to drink (this does not mean chemically pure — it can still contain low levels of dissolved substances such as minerals).
Waste and ground water is made potable through:
- Sedimentation — solid particles settle out at the bottom
- Filtration — remaining suspended solids are removed by passing through filter beds
- Chlorination — chlorine is added to kill microorganisms/pathogens
Sea water cannot be made potable this way because it contains dissolved salts; instead it is made potable by distillation, which removes the dissolved salts by evaporating and condensing the water.
Water used in chemical analysis must not contain any dissolved salts (i.e. it must be pure/deionised water), otherwise the salts could interfere with or contaminate the results of the analysis.
Must Know
- Pure substance (chemistry) = a single element/compound; pure substances have a sharp melting point, mixtures melt over a range
- Filtration separates insoluble solid from liquid; crystallisation separates soluble solid from solution
- Simple distillation separates a solvent from a solution; fractional distillation separates multiple miscible liquids by differing boiling points
- Paper chromatography separates soluble substances using a mobile phase (solvent) moving through a stationary phase (paper); pure substance = 1 spot, mixture = multiple spots
- Rf value = distance moved by substance ÷ distance moved by solvent, used with reference substances to identify components
- Waste/ground water made potable via sedimentation, filtration, chlorination; sea water made potable by distillation; analysis water must have no dissolved salts
That's the notes covered.
Carry on to the next subtopic.