Cambridge O Level Chemistry · Syllabus 5070 · Experimental Techniques and Chemical Analysis
Crystallisation
What is Crystallisation?
A purification method that obtains a dissolved solid as crystals. The solution is heated to evaporate some of the solvent until it reaches the point of crystallisation, then allowed to cool. As the temperature falls the maximum concentration that can stay dissolved falls, so the excess solute separates as crystals. These are filtered off, washed with a little cold solvent and dried. Soluble impurities stay behind in the remaining solution.
This definition is part of the Experimental Techniques and Chemical Analysis chapter in Cambridge O Level Chemistry.
Crystallisation in context
Experimental techniques and chemical analysis cover how a chemist gathers and interprets evidence: choosing the right apparatus for a measurement, running a titration to find a reacting volume, using paper chromatography to separate and identify substances, separating and purifying mixtures by filtration, crystallisation or distillation, and running the qualitative analysis tests that identify anions, cations, gases and metal ions. A titration uses a volumetric pipette to measure a fixed volume and a burette to add a variable volume until an indicator shows the end-point, giving a titre as final reading minus initial reading. Chromatography separates soluble substances by how far they travel in a solvent, measured as the retention factor, Rf.
Common mistakes with Crystallisation
- 27. “Use a separating funnel to separate the mixture.” Why it fails A separating funnel is not one of the five methods named in statement 12.4.1, so it is not the expected answer — and it only works at all for liquids that do not mix. Correct Choose from a suitable solvent, filtration, crystallisation, simple distillation and fractional distillation, and justify the choice from the property data. Check Ethanol and water are miscible, so no layers form; fractional distillation is the method.
Questions students ask about Crystallisation
How should you choose a separation technique on an unfamiliar substance?
Match the technique to the property difference the question gives you: use a suitable solvent or filtration when solubility differs, crystallisation to recover a dissolved solid, simple distillation to separate a solvent from a dissolved solid or non-volatile component, and fractional distillation when two or more miscible liquids have different boiling points. A full-credit answer names the method, states the property difference that makes it work, and says what is kept.