ZIMSEC O Level · Combined Science · Form 1
Mixtures can be separated using differences in the physical properties of their parts.
Methods
- Filtration: separates an insoluble solid from a liquid (sand from water). The solid stays on the filter paper as the residue; the liquid passing through is the filtrate.
- Evaporation: obtains a dissolved solid from a solution (salt from salt water).
- Crystallisation: heat a solution until it is saturated, then let it cool so crystals form (for example copper sulfate crystals).
- Simple distillation: obtains a pure solvent from a solution (pure water from sea water). The liquid boils, and the vapour cools and condenses in a condenser.
- Fractional distillation: separates liquids with different boiling points (ethanol from water; crude oil into fractions).
- Chromatography: separates dyes or inks; different dyes travel different distances up the paper.
- Decanting and using a magnet are simple methods for some mixtures (iron filings from sulfur).
Worked example: How would you obtain pure dry salt from a mixture of salt and sand?
1. Add water and stir: the salt dissolves, the sand does not.
2. Filter: the sand stays on the filter paper.
3. Evaporate the filtrate to obtain salt crystals.
Exam tip: Name the method AND say why it works, e.g. “filtration, because sand is insoluble”.
Check yourself
- Which method separates the colours in black ink?
- Why does water collect in the condenser during distillation?
- Name the solid left on the filter paper.
Show answers
- Chromatography.
- The steam is cooled by the cold water around the condenser and condenses back to liquid.
- The residue.
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