Why does slow crystallization usually produce purer crystals?

Slow crystallization is essential for achieving high purity because it allows for the orderly arrangement of molecules within the crystal lattice. When solidification occurs rapidly, foreign material and impurities can easily become trapped within the solid matrix.1

By maintaining a slow rate of growth, the process ensures that particles have sufficient time to arrange themselves into the characteristic pattern of the crystalline solid, effectively excluding impurities that might otherwise be incorporated into the structure. Rapid growth, by contrast, is more likely to lead to instabilities and defects, such as dendritic growth or the entrapment of foreign substances. Consequently, controlling the speed of precipitation or cooling is a fundamental principle in separation and purification to ensure the resulting crystals are as pure as possible.123