Crsytallisation

3.1: Crystallisation Method

Crystallisation separates a dissolved Solid from a solution as well-formed crystals.
One way to carry out crystallisation is to heat a solution to evaporate off most of the solvent. The hot, saturated solution is then allowed to cool. As it cools, crystals of the dissolved solid (solute) form. The impurities remain in the solution. The main steps in purifying a solid by crystallisation are shown in this video.


Crystallisation occurs because the solutbility of most solute decreases as the temperature decreases. That is, less solute can dissolve in a solution at a lower temperature than at a higher temperature. As hot solution cools, it eventually becomes saturated, that is, it can hold on more solute. The extra solute, that cannot be dissolved, seaparates as pure crystals. Impurities, if present in small amounts, remain in the solution.

3.2: Sidenote

Salt is unusual in that the water solvent must be completely evaporated from the solution to obtain the crystals of salt. Crystallisation from a solution is the most common method used by chemists to purify solids. Pure sugar is obtained this way.

Some solids can be purified by melting them. The hot liquid is then cooled slowly. Pure crystals form as the liquid freezes. This is crystallisation without a solvent. This is how crystals of rock, such as quartz, are produced from molten rock in the earth.

This is a picture of a quartz: