What Is Crystallisation?
Crystallisation is a separation technique used to obtain a solid solute from a solution. It works because solubility decreases as a solution cools or as solvent evaporates.
When the solution becomes saturated (holds the maximum amount of dissolved solute) and then more solvent is removed, solute particles begin to arrange themselves into regular patterns — crystals.
How to Do a Crystallisation
- Heat the solution — gently evaporate some of the water so the solution becomes more concentrated
- Test for saturation — dip a cold glass rod into the hot solution; if crystals form on the rod, the solution is saturated
- Allow to cool slowly — large, well-formed crystals grow slowly
- Filter the crystals from the remaining solution (mother liquor)
- Pat dry with filter paper (or dry in a warm oven at low temperature to avoid decomposition)
If you evaporate all the water too quickly (by boiling to dryness), you get a fine powder of tiny crystals with impurities — not the large pure crystals obtained by slow, careful crystallisation.
Solubility and Temperature
Most solids become more soluble at higher temperatures (they dissolve more easily in hot water). Cooling a hot, saturated solution causes excess solute to crystallise out because less can remain dissolved.
Example: copper sulfate is very soluble in hot water. As the solution cools, beautiful blue crystals of copper(II) sulfate pentahydrate (CuSO₄·5H₂O) form.
Why Grow Crystals Slowly?
Slow crystal growth allows particles to align perfectly into the regular, repeating lattice structure. Rapid cooling or fast evaporation traps impurities and creates small, imperfect crystals.
Types of Crystals
Different ionic compounds form characteristic crystal shapes:
| Substance | Crystal shape | Colour |
|---|---|---|
| Copper(II) sulfate | Triclinic (spiky, angular) | Blue |
| Sodium chloride | Cubic | White |
| Potassium alum | Octahedral | Colourless |
Applications
- Salt production: sea water is evaporated in solar pans; salt crystals are harvested
- Sugar refining: raw sugar solution is crystallised to produce pure white sugar
- Pharmaceutical industry: medicines are purified by recrystallisation
- Gemstone formation: minerals like quartz and calcite form crystals underground over thousands of years
Try growing your own crystals! Dissolve as much salt or alum (from a pharmacy) as possible in hot water, let it cool slowly, then hang a seed crystal on a thread in the saturated solution. Large crystals can grow over several days.
Real world: Sugar refining — crystallising pure white sugar from cane juice
Raw sugar cane juice is filtered and then boiled under reduced pressure (vacuum evaporation) to concentrate it without burning the sugar. When the solution becomes supersaturated, seed crystals are added to trigger controlled crystallisation. Sucrose molecules arrange into the characteristic cubic crystals of table sugar. The brown liquid (molasses) is spun off in a centrifuge. The result is the white crystalline sugar in your kitchen — a 99.9% pure substance produced from a complex plant juice by evaporation and crystallisation.
Real world: Pharmaceutical recrystallisation purifies medicines
When a drug is synthesised in the lab, it often contains small amounts of chemical impurities from side reactions. Recrystallisation is used to purify it: the crude drug is dissolved in a hot solvent, then cooled slowly. The desired drug, being more abundant, crystallises first in a pure form. Impurities remain dissolved in the mother liquor (the remaining solution). The crystals are filtered off, washed, and dried. Drug companies use this technique to achieve the >99.9% purity required before a medicine can be approved for patients.
Real world: Mineral crystals form naturally underground over millions of years
Quartz, calcite, emerald, and salt (halite) all form by crystallisation from supersaturated solutions deep underground. Mineral-rich water percolates through rock crevices. As pressure and temperature change, the water cools or evaporates, triggering crystallisation. Slow cooling over thousands or millions of years creates large, perfectly formed crystals — the large quartz crystals in geodes or the giant selenite crystals in Mexico's Cave of Crystals (up to 12 metres long). The same chemistry as a school crystallisation experiment, but on geological timescales.
Where do you see this in real life? White table sugar crystallised from cane juice, pharmaceutical drugs purified by recrystallisation, salt harvested from solar evaporation pans, quartz and gemstone crystals formed slowly underground, copper sulfate growing beautiful blue crystals when a hot saturated solution is cooled — crystallisation is how nature and industry produce pure solids from impure solutions.