Crystallisation — Chemistry, 11–13 years
How a dissolved substance can form a solid with a regular crystal shape. Chemistry, 11–13 years.
Growing crystals
When a solution can no longer hold all of a dissolved substance, some particles leave the liquid and arrange themselves into a solid crystal. Slow cooling or evaporation gives them time to line up, so the crystal faces can become regular.
Why crystallise
A dissolved solid cannot be caught by ordinary filtering because its tiny particles pass through with the liquid. Crystallisation solves this problem by turning the solid into larger, separate crystals, and careful conditions can leave many dissolved impurities behind.
Crystals from salt water
At about 20 °C, roughly 36 g of table salt can dissolve in 100 g of water. If that solution is left in a shallow dish, water evaporates; after about half the water is gone, only about 18 g of salt can remain dissolved, so the rest forms crystals. Slow evaporation gives clearer crystals.
Why speed can spoil crystals
It is tempting to boil away all the water quickly, because that seems to finish the job sooner. Rapid evaporation can trap impurities, make many tiny crystals or cause splashes, while slow cooling or evaporation lets particles arrange more neatly and safely.
Crystals in the world
Salt makers evaporate seawater in shallow ponds so salt crystals can be collected. Sugar is also purified by crystallisation, and laboratories use the same idea to obtain a solid chemical from a solution without carrying all the liquid along.
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