Topic 8 of 8
Separation and recovery
Choose what to retain and how to recover it.
O-Level 6092 (2026) / SEC G3 K324 (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.
Rf is the distance moved by the spot centre divided by the solvent-front distance, both measured from the baseline; it has no unit. Compare values only under the same conditions.
Worked example
Interpret chromatography conservatively
A spot moves 3.6 cm while the solvent front moves 6.0 cm. A reference compound in the same run has a spot at the same position.
- Rf = 3.6 / 6.0 = 0.60.
- Matching position under the same conditions is consistent with the reference identity.
- It does not prove identity by itself: different substances can have the same Rf, and an unresolved mixture can appear as one spot.
Rf = 0.60; use the comparison as evidence with the stated limitation.
For a soluble salt from an acid and an insoluble base or carbonate, add the solid until it is in excess, filter off unused solid, then crystallise the filtrate. For two soluble reactants giving an insoluble salt, mix, filter the precipitate, wash and dry. For an acid and soluble alkali, use titration to establish the reacting volumes, then repeat without indicator before crystallisation. Avoid evaporating a heat-sensitive salt to complete dryness.