Recrystallisation — Chemistry, 14–17
Purify an impure solid by dissolving it hot and forming cleaner crystals as it cools. Chemistry, 14–17 years.
Cleaning a solid with crystals
Recrystallisation separates a desired solid from small amounts of impurity. Choose a solvent that dissolves the substance well when hot but only a little when cold. The desired material comes out as crystals during cooling, while much of the impurity stays in the liquid or is removed first by filtration.
Why not just filter it?
Filtering catches solid particles, but it cannot remove an impurity mixed into the same solid crystals. The problem is to separate substances that look like one powder. Recrystallisation uses their different solubilities at different temperatures, a method developed from practical ways of purifying medicines and chemicals.
Purifying benzoic acid
Suppose 6.00 g of benzoic acid contain impurities. Add about 100 mL of hot water: up to 6.8 g dissolves. Cool slowly, then filter; at 20 °C, about 0.29 g remains dissolved. So the maximum crystal mass is 6.00 − 0.29 = 5.71 g, or about 95% recovery if the impurities were insoluble.
Too much solvent loses product
A reasonable mistake is to add lots of solvent so every piece dissolves easily. But the extra solvent still holds more product when cold, so fewer crystals are recovered. Use the smallest hot volume that dissolves the solid, and cool slowly; fast cooling can trap impurities inside the growing crystals.
Purifying real products
Recrystallisation is used after making medicines, dyes and other organic chemicals in the laboratory. A product may have the right formula but still contain traces of starting materials or side-products. Purity matters because a small impurity can change colour, melting point, biological effect or safety.
Keep exploring
Other languages
Loading MyLeoNes™…