Final answer:
Crystallization is the process of solutes in a saturated solution forming solid crystals upon cooling. Recrystallization is used to purify compounds, involving dissolving the impure substance in a hot solvent and then cooling to precipitate pure crystals. Supersaturated solutions can also crystallize when a seed crystal is introduced.
Step-by-step explanation:
Crystallization is a process where dissolved solutes return to a solid state. This occurs in a saturated solution, which contains the maximum amount of solute capable of dissolving at a given temperature. By contrast, an unsaturated solution has less than this maximum amount. Heating a solution increases the solubility of the solute, allowing more to dissolve and potentially creating a supersaturated solution when it is cooled. In the cooling process, the solubility decreases, causing excess solute to precipitate out as pure crystals.
During recrystallization, a hot solvent is used to dissolve impure crystals. Upon cooling, the solute's solubility declines, and pure crystals form, leaving impurities in the solution. This method is frequently employed to purify chemical compounds. Melting and freezing also relate to crystallization as heating a substance increases its particles' energy, leading to melting. This goes on until all the solid has melted and only resumes the temperature rise when melting is complete.
In cases of substances that do not crystallize immediately when saturated, like sodium acetate trihydrate, providing a seed crystal can initiate rapid crystallization, releasing heat in an exothermic reaction.