Crystallisation – IGCSE Chemistry Definition

Crystallisation is an essential step in preparing pure, dry salt crystals from solution. It appears in salt preparation methods (Paper 4) and as a practical technique (Paper 6). The method description is worth 2–3 marks, and examiners expect specific procedural detail.

The 0620 definition

Crystallisation is the process of forming solid crystals from a solution, usually by evaporating the solvent so that the solution becomes saturated and crystals form.

The standard method

1. Heat the solution gently using an evaporating basin on a water bath or tripod with gauze
2. Evaporate until the point of crystallisation — test by dipping a glass rod into the solution; if crystals form on the rod when cooled, stop heating
3. Allow the hot, concentrated solution to cool slowly (slow cooling gives larger crystals)
4. Filter to collect the crystals
5. Wash crystals with a small amount of cold distilled water (removes impurities without dissolving much product)
6. Dry between sheets of filter paper, or in a warm oven (not too hot — some crystals lose water of crystallisation)

Where crystallisation fits in salt preparation

Method 1: Acid + excess insoluble base

1. Add excess insoluble base to warm acid
2. Filter off the excess solid (filtration)
3. Crystallise the filtrate to obtain salt crystals

Method 2: Titration

1. Find exact volumes by titration
2. Repeat without indicator
3. Crystallise the resulting solution

Point of crystallisation

This is the moment when the solution is concentrated enough for crystals to start forming. Heating beyond this point risks:

• Splattering (hot solution spitting)
• Decomposition of the salt
• Producing small, impure crystals

Worked exam question

A student has a solution of copper(II) sulfate. Describe how to obtain pure, dry crystals of copper(II) sulfate from this solution. [3]

Mark scheme

Heat the solution gently / evaporate some water [1]; stop heating at the point of crystallisation / when crystals appear on a glass rod [1]; allow to cool, filter, wash with cold distilled water, and dry [1]

Common exam mistakes

• Saying “boil until dry” — this does not produce good crystals and may decompose the salt.
• Forgetting to mention slow cooling — this is what produces large, regular crystals.
• Omitting the washing step — without it, impurities remain on the crystal surfaces.