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K324 / 2027
Experimental Chemistry

Lesson 3 of 6

Separate by solubility and particle size

Recover the intended substance, not just any solid.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can describe separation methods.Syllabus

    Syllabus K324, 1.2(a). Describe separation methods

  • I can choose separation methods from properties.Syllabus

    Syllabus K324, 1.2(b). Choose separation methods from properties

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.

Separate a sand-and-salt mixture
  1. Add water and stir

    Salt dissolves; sand remains insoluble.

  2. Filter

    Sand is the residue; salt solution is the filtrate.

  3. Wash and dry the sand

    Washing removes remaining salt solution.

  4. 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.

Pure only

Sublimation separates a solid that changes directly to vapour from one that does not. On warming an iodine/sand mixture, iodine vapour forms and deposits as solid iodine on a cooler surface while sand remains. Choose gentle controlled heating and avoid inhaling vapour. This is a property-based separation, not melting followed by boiling.

Check your understanding

A clear sugar solution passes through filter paper unchanged. Why?

  1. Sugar melts in water, so it becomes a liquid that soaks through the paper.
  2. The sugar is dissolved, so its particles pass through the paper with the water.
  3. The filter paper is too coarse; a finer paper would hold back the sugar.
Show the answer

The sugar is dissolved, so its particles pass through the paper with the water.

Filtration separates insoluble solids from a liquid. Dissolved sugar particles are small and spread through the water, so evaporation or crystallisation is needed to recover the sugar.