Separation Techniques — Elements, Compounds and Mixtures | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Definitions and separation techniques tested consistently across both papers.

Mixture	Best method
Insoluble solid + liquid	Filtration
Soluble solid + liquid (want the solid back as crystals)	Crystallisation
Soluble solid + liquid (want the liquid back pure)	Simple distillation
Two or more miscible liquids of different b.p.	Fractional distillation
Coloured substances dissolved in a single solvent	Paper chromatography

Exam tip

Choosing a separation technique

Naming the best separation method for a mixture comes up regularly (often a table of mixtures), so you need to know the matches: insoluble solid in a liquid → filtration; solid from a solution → crystallisation; solvent from a solution → simple distillation; two miscible liquids → fractional distillation; dyes in a solvent → chromatography. The trap is swapping simple and fractional distillation.

Filtration

Separates an solid from a liquid (e.g. sand from water)
Apparatus:
- Folded filter paper in a filter funnel
- Funnel resting on the rim of a beaker (or held in a clamp above it)
The mixture is poured into the funnel; the paper traps solid particles
= solid caught on the paper
= liquid that passes through into the beaker
Centrifugation is an alternative for very fine particles (more common in biology than in chemistry)

Crystallisation

Pulls a solid back out of its solution as well-formed crystals
Works because the solubility of most solids falls sharply on cooling
Procedure:
1. Heat the solution gently to evaporate some solvent and concentrate it
2. To test for saturation, touch the surface with a chilled, clean glass rod. If crystals form on the rod, the solution is saturated

Separates a liquid from a dissolved (e.g. pure water from salt water)
Also isolates a single pure liquid from a mixture whose other components boil much higher
How it works:
- The solution is heated in a round-bottomed flask
- The volatile component vaporises and rises out of the flask
- The vapour passes into a water-jacketed condenser and is cooled back into liquid form
- The pure liquid is collected in a beaker; the solute stays behind in the flask

Separates two or more liquids with different boiling points
- Example: ethanol (b.p. 78 °C) and water (b.p. 100 °C)
- Industrial example: crude oil into petrol, kerosene, diesel etc. (see Topic 11)
Apparatus is simple distillation plus a vertical fractionating column between the flask and the condenser
- Column packed with glass beads (or fitted with plates)
- Provides a large surface area for repeated condensation and re-evaporation
- Each cycle enriches the rising vapour in the lower-boiling component
Procedure (ethanol/water):
- Heat the flask; thermometer at the top of the column reads 78 °C when ethanol vapour reaches it
- Pure (or nearly pure) ethanol distils over; thermometer holds at 78 °C
- When the ethanol is exhausted, the temperature rises towards 100 °C as water starts to come over; stop collecting, or switch flasks

Separates dissolved substances whose solubilities in a single chosen solvent differ from one another
- Typical targets: ink dyes, food colourings, plant pigments
Procedure:
1. Draw a horizontal baseline in pencil about 1–2 cm from one short edge of the chromatography paper
  - Pencil graphite is insoluble; pen ink would itself run up the paper
2. With a fine capillary tube, place one small spot of each sample on the baseline (fresh tube per sample)
3. Stand the paper in a covered beaker with a shallow layer of solvent
  - Solvent level must be below the baseline
4. The solvent climbs the paper by capillary action, carrying dissolved substances with it
  - More soluble in the solvent → travels further
  - Less soluble (sticks to paper) → travels less
5. When the solvent front is near the top, lift out the paper, mark the solvent front in pencil, and let it dry

Elements, Compounds and Mixtures

Exam Frequency Analysis

Filtration

Crystallisation

Simple distillation

Fractional distillation

Paper chromatography