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Lesson: Chapter 3 — Mixtures

3.3. Separating the components 4 of 5

Crystallisation and recrystallisation

Take a solution and keep evaporating it. The solute has nowhere to go but out.

At a given temperature there is a maximum concentration at which a substance can stay in solution — a saturated solution has reached it. If that saturated solution is evaporated further, the concentration of the solute climbs past the maximum that can stay dissolved, and the solute leaves the solution by forming crystals.

Crystallisation

The method of separating out a solid solute from a solution by concentrating it is called crystallisation.

Two industries that run on it

Sugar

Cane is crushed and the juice squeezed out is purified, then its concentration is raised by evaporation. Sugar then leaves the cane juice solution as crystals.

Salt

Producing salt from sea water is another industry that uses crystallisation. In a saltern several of the salts dissolved in the sea water crystallise out — which is the whole of section 3.4.

When the crystals are not pure enough

Crystals obtained this way carry impurities with them. Recrystallisation is used to separate pure compounds out of crystalline solid compounds that contain impurities.

Recrystallisation

The process of dissolving a crystalline solid and turning it back into crystals again is called recrystallisation.

  • The impure solid is dissolved in hot solvent until that solvent is saturated
  • The solution is filtered while still hot, which removes the impure parts that did not dissolve
  • The filtrate is cooled, and pure crystals of the solid form

Why cooling gives pure crystals

The wanted solute is not saturated in the hot solution but becomes saturated as it cools, so it crystallises. The impurities are present only in small amounts, so even in the cool solution they never reach saturation — and they stay dissolved instead of crystallising with it.

Activity 3.3.2 — recrystallising table salt

Method

Get about 50 g of ordinary coarse salt from a shop. Take about 50 cm³ of water at about 90 °C into a beaker and add lumps of salt until the maximum amount has dissolved and it is saturated. Filter the solution through filter paper while it is still hot. Then take this filtrate into a beaker, stand it in a vessel of ice and stir slowly. Look at the crystals that form.