Separation Techniques and Purification
Choosing the right method depends on what kind of mixture you have. Each technique exploits a different property.
1. Choosing the method
| Mixture | Method | Property used |
|---|---|---|
| Insoluble solid + liquid | FILTRATION | particle size |
| Soluble solid from its solution | CRYSTALLISATION / evaporation | solubility |
| Solvent from a solution | SIMPLE DISTILLATION | boiling point |
| Two miscible liquids | FRACTIONAL DISTILLATION | different boiling points |
| Coloured substances / mixtures in solution | CHROMATOGRAPHY | solubility and attraction to paper |
| Two immiscible liquids | separating funnel | density / immiscibility |
2. Filtration
Separates an insoluble solid from a liquid, using filter paper in a funnel.
- The solid trapped in the paper is the RESIDUE
- The liquid passing through is the FILTRATE
Example: sand from water.
Learn the two words — residue and filtrate. Questions ask for them by name.
3. Crystallisation
Separates a soluble solid from its solution.
- Heat the solution to evaporate some solvent, until the point of crystallisation (crystals just start to form at the edge)
- Leave to cool slowly — crystals grow as solubility falls
- Filter off the crystals
- Dry between filter paper or in a warm oven
Do NOT evaporate to dryness — that decomposes some salts and drives off the water of crystallisation, spoiling the crystals.
Slow cooling gives larger, purer crystals.
4. Simple distillation
Separates a liquid (solvent) from a solution, collecting the pure liquid.
Example: pure water from seawater.
How it works: the solution is heated; the solvent evaporates, passes into the condenser, is cooled, and condenses back to a liquid — the distillate. The dissolved solid stays behind.
Key apparatus points:
- The thermometer bulb must be level with the side-arm, to measure the vapour temperature
- Cold water enters the condenser at the BOTTOM and leaves at the top, so the condenser stays full and cools efficiently
- Use anti-bumping granules for smooth boiling
5. Fractional distillation
Separates two or more miscible liquids with different boiling points.
Example: ethanol (78 °C) from water (100 °C); also crude oil and liquid air.
How it works: a fractionating column is fitted above the flask. As vapour rises it repeatedly condenses and re-evaporates, so the substance with the LOWEST boiling point reaches the top first and is collected. There is a temperature gradient — hotter at the bottom, cooler at the top.
The liquid with the LOWEST boiling point distils over FIRST. Watch the thermometer: it holds steady at each substance’s boiling point while that fraction comes over.
6. Chromatography
Separates mixtures of soluble substances, using their different solubilities in the solvent and attraction to the paper.
Method:
- Draw the baseline in PENCIL (ink would dissolve and run)
- Spot the mixture on the baseline
- Stand the paper in solvent with the solvent level BELOW the baseline
- Let the solvent rise, then mark the solvent front and dry
Pencil baseline, and solvent below the baseline — the two most-tested practical points. If the solvent covers the spots, they dissolve into the solvent instead of travelling up.
Interpreting it:
The most soluble substance travels FURTHEST. A pure substance gives ONE spot; a mixture gives SEVERAL. Substances can be identified by comparing with known reference spots.
Rf value:
Rf = distance moved by the SPOT ÷ distance moved by the SOLVENT
Rf is always between 0 and 1 and has no units. Measure both distances from the baseline, and to the centre of the spot.
For colourless substances, use a locating agent to reveal the spots.
7. Other methods
Separating funnel — for immiscible liquids (oil and water). Run off the denser lower layer first.
Magnetism — separates a magnetic solid (iron) from a non-magnetic mixture.
Sublimation — separates a substance that sublimes (iodine, ammonium chloride) from one that doesn’t.
8. Testing for purity
A PURE substance has a SHARP, FIXED melting and boiling point. An IMPURE substance melts and boils over a RANGE, at a LOWER melting point and a HIGHER boiling point.
Chromatography also tests purity: one spot = pure.
9. Mistakes that cost marks
Confusing residue with filtrate.
Evaporating to dryness in crystallisation.
Thermometer in the wrong position.
Water entering the condenser at the top.
Drawing the chromatography baseline in ink.
Solvent level above the baseline.
Measuring Rf from the wrong point, or giving it units.
Choosing the wrong method for the mixture.
Saying an impure substance has a sharp melting point.
Frequently asked questions
What does filtration separate? An insoluble solid from a liquid.
What are the residue and filtrate? The residue is the solid trapped; the filtrate is the liquid that passes through.
Why not evaporate to dryness? It decomposes the crystals and loses the water of crystallisation.
What does simple distillation separate? A solvent from a solution.
Where should the thermometer bulb be? Level with the side-arm, to measure the vapour temperature.
Which way does water flow through the condenser? In at the bottom, out at the top.
What does fractional distillation separate? Miscible liquids with different boiling points.
Which liquid comes over first? The one with the lowest boiling point.
Why is the chromatography baseline drawn in pencil? Ink would dissolve and run up the paper.
How do I calculate Rf? Distance moved by the spot ÷ distance moved by the solvent — between 0 and 1, no units.
Quick revision checklist
- I can choose the right method for any mixture
- I know residue and filtrate
- I can describe crystallisation without evaporating to dryness
- I know the thermometer and condenser details for distillation
- I know how fractional distillation separates by boiling point
- I know the lowest boiling point distils first
- I use a pencil baseline and keep solvent below it
- I can calculate and interpret Rf
- I know one spot means pure
- I know about separating funnels, magnetism and sublimation
- I know pure substances have sharp melting points
These notes cover separation techniques and purification in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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