Topic 7 of 7
Separation and recovery
Choose what to retain and how to recover it.
SEC G2 Science K223 / K225 (2027)
Recover the substance you need
Choose a process from solubility, volatility and thermal stability.
| Task | Method | What is recovered |
|---|---|---|
| Remove an insoluble solid from a liquid | Filtration | Solid is the residue; liquid passing through is the filtrate |
| Obtain a dissolved solid that crystallises on cooling | Concentrate, cool, filter crystals, wash sparingly and dry | Crystals; some solute remains dissolved in the mother liquor |
| Recover a solvent | Simple distillation | Condensed vapour from the distillate; monitor the apparatus correctly |
| Compare soluble mixture components | Paper chromatography | Separated spots, compared under the same conditions |
For paper chromatography, use a pencil baseline above the solvent, small concentrated spots and a covered vessel where appropriate. Remove the paper before the solvent reaches the top and mark the solvent front immediately.
Check your understandingWhy wash a filtered insoluble precipitate with a small amount of distilled water?Think it through, then reveal the answer
To remove soluble solution and contaminants clinging to it. The wash should not dissolve an appreciable amount of the desired precipitate; consider solubility when choosing the washing conditions.
G2 practical work includes crystallisation of salts, filtration, paper chromatography and distillation. Describing the full preparation of a pure salt is not required.