Crystallisation

The formation of solid crystals from a homogeneous or uniform solution is called crystallisation. It is generally used to separate a solid that has dissolved in a liquid and made a solution. For crystallization to take place, a solution must be “supersaturated”, a state in which the liquid or solvent contains more dissolved solute than can ordinarily be retained at that temperature.
 
Substances like sugar, salt, urea, copper sulphate form crystals. The solution is warmed in an open container, allowing the solvent to evaporate, leaving a saturated solution. A solution that has as much solid dissolved in it as it can possibly contain is called a saturated solution. As the saturated solution is allowed to cool, the solid will come out of the solution and crystals will start to grow. The crystals can then be collected and allowed to dry. A solid that does not dissolve in a liquid can be separated by filtration. Crystallisation is a physical change as the substance undergoes crystallisation retains its chemical properties.
 
The size of crystals depends on the rate of cooling. Fast cooling will result in a large number of small crystals. Slow cooling will result in a smaller number of large crystals. Lets perform an activity to observe how crystals can be grown.