Question

the theory behind recrystallization. How does it work on the molecular scale? Segregation and weakening of the crystal lattice? why it is important with unknowns ?

Answer 1

The principle behind recrystallization is that the amount of solute that can be dissolved by a solvent increases with temperature. In recrystallization, a solution is created by dissolving a solute in a solvent at or near its boiling point. At this high temperature, the solute has a greatly increased solubility in the solvent, so a much smaller quantity of hot solvent is needed than when the solvent is at room temperature. When the solution is later cooled, after filtering out insoluble impurities, the amount of solute that remains dissolved drops precipitously. At the cooler temperature, the solution is saturated at a much lower concentration of solute. The solute that can no longer be held in solution forms purified crystals of solute, which can later be collected.

Recrystallization works only when the proper solvent is used. The solute must be relatively insoluble in the solvent at room temperature but much more soluble in the solvent at higher temperature. At the same time, impurities that are present must either be soluble in the solvent at room temperature or insoluble in the solvent at a high temperature.