In many people's opinion, crystals are beautiful specimens of nature (Figure 3.1). Many crystals have long lines and edges, form geometric patterns, and have sheer surfaces that reflect light and cause them to sparkle. The macroscopic appearance of a crystal is a direct result of its microscopic structure, where a regular, repeating unit forms a three-dimensional crystal lattice. The uniformity and structural repetition differentiate a crystal from an amorphous solid. For this reason, there is a difference between precipitation (the rapid formation of a solid), and crystallization (the slow growth of a solid with regular microscopic structure). Precipitated solids tend to have lower purity than crystals, which is why crystallization is of interest to organic chemists (and geologists!).
Crystallization is used in the chemistry laboratory as a purification technique for solids. An impure solid is completely dissolved in a minimal amount of hot, boiling solvent, and the hot solution is allowed to slowly cool. The developing crystals ideally form with high purity, while impurities remain in the saturated solution surrounding the solid (called the "mother liquor"). The crystallized solid is then filtered away from the impurities. A diagram of the crystallization procedure is shown in Figure 3.2.
As an impure solid may have originally formed through partial crystallization, some call this procedure a "recrystallization", as the solid is allowed to crystallize a second time under more careful conditions. In this chapter the technique will be referred to as crystallization, which does not imply any previous process.