Crystallisation is the formation of crystals from a hot, saturated solution as it cools, used to obtain a pure, dry sample of a soluble salt.
Malay: Penghabluran · Chinese: 结晶
Crystallisation is the formation of crystals from a hot, saturated solution as it cools, and it is the standard way to recover a pure, dry sample of a soluble salt. As the hot solution cools, it can hold less dissolved salt, so the excess comes out of solution as regular, well-formed crystals, leaving impurities behind in the liquid.
A worked example gives the method. After reacting an acid with excess base and filtering off the unreacted solid, you heat the salt solution to evaporate some water until it is saturated, tested by dipping in a cold glass rod and seeing crystals form on it. You then stop heating and let the concentrated solution cool slowly. Crystals grow as it cools; you filter them out, wash with a little cold distilled water, and dry them between filter papers. The result is a pure, dry salt such as hydrated copper(II) sulfate.
The confusion to avoid is between crystallisation and evaporating to dryness. You must not evaporate all the water for a salt like copper(II) sulfate, because boiling it dry drives off the water of crystallisation and can decompose the salt, ruining the crystals. Heat only to saturation, then cool. A second slip is confusing crystallisation with precipitation: precipitation makes an insoluble salt appear instantly on mixing two solutions, while crystallisation slowly grows crystals of a soluble salt on cooling.
Within this chapter crystallisation is the final, purifying step in preparing a soluble salt, following neutralisation or the reaction of an acid with a metal, base or carbonate. Our teachers show you the cold-glass-rod saturation test so you stop heating at exactly the right moment.
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