This video introduces the concepts of supersaturation, nucleation and crystallisation. The amount of solute that can dissolve in a liquid, i.e. its solubility, increases with temperature. Up to a point, warming a solution makes it possible to dissolve more salt. Using this idea, heat up a beaker of copper sulphate solution, gradually add copper sulphate and dissolve it. This is called supersaturation, since the solution now holds more salt dissolved in it than is possible at room temperature. As the liquid cools, the additional salt will recrystallize and come out of the solution. By providing a rough surface such as a length of string, a process called nucleation occurs, in which a small amount of salt first settles, then grows into a crystal as more molecules deposit on the site. By the time the solution has cooled to room temperature, all the extra salt dissolved is deposited in crystal form along the length of string.

*** Warning: Be careful in handling copper sulphate as it is irritating to the eyes and skin, and also can be harmful if swallowed. ***