Skip to notes
wise otter.Chemistry
Contents
K326 / K328 / 2027
Experimental Chemistry

Lesson 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)

  • I can describe separation methods.Syllabus

    Syllabus K326 / K328, 1.2(a). Describe separation methods

  • I can choose methods from properties.Syllabus

    Syllabus K326 / K328, 1.2(b). Choose 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.

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.