2.2 (b) Methods of purification
-Filtration: Mixture goes in a funnel with filter paper, into a flask. Residue is insoluble and stays at top. Filtrate goes through.
Crystallisation: Some water in the solution is evaporated so the solution becomes more concentrated. One drop is placed on a microscope slide to check if crystals are forming. The solution is left to cool and crystallise. Crystals are filtered to remove solvent.
Distillation:
Simple distillation (left picture below) evaporates a solvent from a solution.
Fractional distillation (right picture below) removes a liquid from a mixture of liquids, because the liquids have different boiling points. Used to separate substances in crude oil and get ethanol from the products of fermentation.
- mixture is heated to evaporate the substance with the lowest boiling point
- some of the other liquid(s) will evaporate A mixture of gases condense on the beads in the fractional column. So the beads are heated to the boiling point of the lowest substance in this case, so that the substance being removed cannot condense on the beads. The other substances continue to condense and will drip back into the flask. The beaker can be changed after every fraction.
Using a suitable solvent:
| Solvent | It dissolves |
| water | see “Soluble salts”, sugar |
| white spirit | gloss paint |
| propanone | grease, nail polish |
| ethanol | glues, printing inks, scented substances in perfumes and
aftershaves |
Choosing a suitable separation method:
| Method of separation | Used to separate |
| filter | a solid from a liquid |
| evaporate | a solid from a solution |
| crystallise | a solid from a solution |
| distil | a solvent from a solution |
| fractional distillation | liquids from each other |
| chromatography | different substances from a solution |