Crystallization | Organic Chemistry Lab Techniques

Learn how crystallization uses temperature-dependent solubility to purify solids. The instructor explains the thermodynamic basis for solubility changes with temperature and introduces the general technique of dissolving material in hot solvent and cooling slowly.

Choose a solvent that dissolves the substance well at high temperature but poorly at low temperature. Evaluate solubility curves for candidate solvents and consider using mixed solvents like methanol-water or ethanol-water combinations to achieve ideal temperature-dependent solubility.

Add the solute to the selected solvent at an elevated temperature, using just enough solvent to dissolve all the product. Heat until the solution reaches saturation, optionally filtering hot to remove insoluble impurities before proceeding to crystallization.

Lower the solubility of the dissolved solute by cooling the solution, adding an anti-solvent, evaporating solvent, or performing a precipitation reaction. The most common method is controlled cooling to slowly reach the metastable limit where nucleation begins.

Watch as the solution becomes supersaturated and crystals begin to nucleate and grow on the walls or introduced nucleation sites. The metastable limit marks the point where dissolved molecules organize into solid crystal structures, building larger crystals over time.