Topic 3 of 6
Separate by solubility and particle size
Recover the intended substance, not just any solid.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Separate by solubility and particle size
Recover the intended substance, not just any solid.
Filtration separates an insoluble solid from a liquid. The residue remains on the filter paper; the filtrate passes through. Dissolved salt particles pass through with water, so filtering salt solution does not remove the salt. Wash a residue with a small amount of suitable solvent to remove soluble impurities, then dry it.
- Add water and stir
Salt dissolves; sand remains insoluble.
- Filter
Sand is the residue; salt solution is the filtrate.
- Wash and dry the sand
Washing removes remaining salt solution.
- Recover salt from the filtrate
Evaporate water for a heat-stable salt, or concentrate and crystallise.
For crystallisation, heat a solution gently to remove some solvent until it is nearly saturated, then let it cool. Crystals form as solubility decreases. Filter, wash with a little cold solvent and dry. Evaporation to dryness removes all solvent and suits a stable dissolved solid; strong heating may decompose some compounds. Crystallisation often leaves soluble impurities in the remaining solution.