Separating MixturesOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is a mixture?
A mixture consists of two or more elements or compounds that are not chemically combined together. Unlike a compound, the substances in a mixture keep their own individual chemical properties, and can be separated back out using physical methods, without any chemical reaction taking place.
Section 2
How does filtration separate mixtures?
Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper held in a funnel: the liquid (and anything dissolved in it) passes through as the filtrate, while the insoluble solid is trapped as the residue.
Filtration can separate sand (insoluble) from a mixture of sand and water: the sand is trapped as the residue, and the water passes through as the filtrate.
Section 3
How does crystallisation separate mixtures?
Crystallisation is used to obtain a solid solute from a solution, for example separating a dissolved salt from water. The solution is gently heated to evaporate off some of the solvent, forming a saturated solution. As it is then left to cool, crystals of the solute form and can be filtered out and dried.
Section 4
How does distillation separate mixtures?
Distillation separates a liquid from a solution (or separates two liquids with different boiling points) by using differences in boiling point.
- The mixture is heated; the substance with the lower boiling point evaporates first.
- The vapour travels away from the flask and is cooled in a condenser.
- The vapour condenses back into a liquid and is collected separately, leaving the other substance(s) behind.
This is useful both for obtaining a pure solvent from a solution, and for separating a mixture of miscible liquids.
Section 5
Which method should be chosen for a given mixture?
| Mixture type | Best method |
|---|---|
| Insoluble solid + liquid | Filtration |
| Dissolved solid, want the solid | Crystallisation |
| Dissolved solid, want the pure liquid/solvent | Distillation |
| Two miscible liquids with different boiling points | Distillation |
Choosing the right physical method depends on what you want to keep (the solid or the liquid) and how the substances are combined (dissolved vs. simply suspended).
In exam answers, always justify the method by referring to a property difference - e.g. 'distillation works because the two liquids have different boiling points' - a named method alone rarely gets full marks.
Must Know
- A mixture contains substances that are not chemically combined and can be separated by physical methods.
- Filtration separates an insoluble solid (residue) from a liquid (filtrate).
- Crystallisation obtains a solid solute from a solution by evaporating solvent and cooling to form crystals.
- Distillation separates liquids, or a liquid from a solution, using differences in boiling point.
- The right method depends on whether the mixture is a solid/liquid combination, a solution, or a mixture of liquids.
That's the notes covered.
Carry on to the next subtopic.