Crystallisation

Summary: Crystallisation is a separation technique used to obtain a pure soluble solid from its solution by evaporating some solvent and then allowing the solution to cool. Slow cooling produces larger, purer crystals. Tags: igcse chemistry separation crystallisation keyword Created: 2026-07-14 Last Updated: 2026-07-14

Definition

Crystallisation is a physical separation technique that produces solid crystals from a solution. As the solution cools, solubility decreases and the dissolved solid comes out of solution as regularly-structured crystals.

Method

1. Heat the solution to evaporate some solvent (NOT to dryness)
2. Stop heating at the crystallisation point — when crystals first appear on cooling / a glass rod dipped in gives crystals
3. Allow to cool slowly at room temperature
4. Filter the crystals from the remaining solution
5. Wash with a little cold distilled water (removes soluble impurities; cold water minimises re-dissolving)
6. Dry between filter paper or in a warm oven

Key Exam Points

• Slow cooling = larger, purer crystals (atoms have time to arrange in the crystal lattice)
• Fast cooling = smaller, less pure crystals (may trap impurities)
• Cold distilled water for washing (not hot — would dissolve the product)
• Do NOT evaporate to dryness — some salts decompose on strong heating; you get a powder, not crystals

See Also

• Separation Techniques — All separation methods
• Salt Preparation Methods — Crystallisation used in salt preparation
• Water of Crystallisation — Water molecules in the crystal lattice
• Experimental Techniques — Practical techniques overview
• IGCSE-Chem-Index