The big idea: Because the parts of a mixture are not chemically bonded, they can be separated by physical means.
The method you choose depends on a physical property in which the components differ — for example their particle size, their solubility, or their boiling point.
There is no single 'best' method — match the technique to the property that differs.
Solid + liquid
- insoluble solid → filtration
- dissolved solid → evaporation / crystallisation
Liquid + liquid
- different boiling points → distillation
- (immiscible liquids → separating funnel)
Mixture in solution
- coloured / dissolved components → chromatography
- separates by solubility
Soluble or insoluble?: The first thing to decide for a solid is whether it dissolves in the chosen solvent.
- Insoluble in the liquid → filter it out. - Soluble (dissolved) → evaporate / crystallise the solution to recover it.
Each technique works because the components differ in one physical property. Learn the technique together with the property it exploits — that is exactly what the exam tests.
| Technique | Separates | Physical property exploited |
|---|---|---|
| Filtration | an insoluble solid from a liquid | particle size (the solid is too big to pass through the filter paper) |
| Evaporation / crystallisation | a dissolved solid from its solution | boiling point — the solvent boils off, leaving the solid behind |
| Distillation | a liquid from a liquid (or a liquid from a dissolved solid) | difference in boiling point |
| Chromatography | the components of a mixture in solution (e.g. coloured dyes) | difference in solubility / attraction to the paper |
Worked thinking — a mixture of liquid hydrocarbons: Petrol is a mixture of liquid hydrocarbons that do not react but have different boiling points.
The property that differs is boiling point, so the right technique is distillation (fractional distillation) — not filtration (nothing is an insoluble solid) and not chromatography.
Purifying a solid product: If a reaction makes a solid product that came out of solution, you purify it by crystallisation: dissolve it in hot solvent, let it cool so pure crystals form, then filter off the crystals and leave the impurities behind in the solution.
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Chromatography separates the dissolved components of a mixture. A spot of the mixture is placed on a start line; a solvent then rises up the paper (or TLC plate) and carries each component a different distance, because each has a different solubility / attraction to the paper. The component that is more soluble in the solvent travels further.
Each component is identified by its retardation factor, Rf — the distance its spot moved compared with how far the solvent moved.
- retardation factor (no units, 0 < R_{f} < 1)
- from the start line to the centre of the spot (cm)
- from the start line to the solvent front (cm)
Always less than 1: The spot can never travel further than the solvent front, so Rf is always between 0 and 1 and has no units. A larger Rf means the component is more soluble in the solvent (it moved further).
Worked example — calculating Rf
On a chromatogram, a spot of dye Y travels 3.6 cm from the start line while the solvent front travels 8.0 cm. Calculate the Rf value of dye Y.
Solution
- Formula first — write the relationship before the numbers:
- Substitute the two measured distances (both in cm, so the units cancel):
- Work it out — the answer is a ratio with no units:
Final answer
Rf = 0.45 (no units, as expected between 0 and 1).
How this is tested: S1.1.3 shows up two ways.
- Paper 1A (MCQ): identify the best technique for a given mixture, or determine an Rf value by reading distances off a chromatogram. - Paper 1B / Paper 2: a longer suggest / outline a sequence question — typically separating a multi-component solid mixture and recovering each part.
Scoring the sequence: For 'separate and recover X, Y and Z', name a technique and the property it uses for each part, in a sensible order. Use a unique property to pick one component off at a time (e.g. magnetism first, then solubility).
IB-style question — separating sand, salt and iron
A solid mixture contains sand (insoluble), table salt (soluble in water) and iron filings. Outline a sequence of steps to separate and recover all three components as pure solids. [4]
How to score the marks
- Mark 1 — remove the iron. Use a magnet to attract and remove the iron filings (iron is magnetic; sand and salt are not).
- Mark 2 — dissolve the salt. Add water and stir: the salt dissolves but the sand does not (difference in solubility).
- Mark 3 — filter. Filter the mixture: the insoluble sand stays as the residue on the filter paper; the salt solution passes through as the filtrate.
- Mark 4 — recover the salt. Evaporate / crystallise the filtrate to drive off the water and leave solid salt behind.
Final answer
Magnet → iron. Add water, dissolve salt. Filter → sand (residue). Evaporate the filtrate → salt.