Crystallization is a technique for purifying solids that contain small amounts of impurities. This technique is based on the fact that both the solid and the impurities may dissolve in a given solvent, but not to the same extent.
Solubility is a function of concentration. If we keep the amount of solvent constant and gradually increase the amounts of solute, eventually we approach a limit beyond which the solute can no longer dissolve because it’s too much for the amount of solvent available.
Another factor important in solubility is temperature. The amount of solute that a given solvent can dissolve normally increases as a function of temperature, although there are always a few exceptions.
The third factor that comes into play in solubility is polarity. In introductory chemistry courses we learn about the “like dissolves like” principle. This makes direct reference to polarity. More polar substances dissolve better in polar solvents (for example alcohol in water), whereas less polar substances dissolve better in less polar solvents (for example butter in cooking oil).
All or any of these three factors can be manipulated to change the solubility of a substance. For example, if we have a solution of salt in water, we can cause some of the salt to come out of solution by evaporating some of the water (changing the concentration), lowering the temperature of the water, or adding a less polar solvent such as rubbing alcohol (decreasing the polarity).
In the technique of crystallization, the most commonly manipulated parameter is temperature. First we dissolve the impure solid in a solvent that dissolves both, solid and impurities. We try to dissolve the maximum amount of sample in the minimum amount of solvent at high temperature (at or near the boiling point of the solvent). After the sample is completely dissolved, we allow it to cool down to room temperature.
The amount of solute that the solvent can dissolve at low temperature is lower than at high temperature. Since the concentration of the main solid is very high, as the solution cools down some of the solid will come out of solution, hopefully in the form of pure crystals. The impurities, being present in small amounts, should stay in solution because of their much lower concentration compared to the main solid. When these conditions are met, we have a successful crystallization set up. Study carefully figure 11.1 on p. 648 of the textbook in connection with these points.
Two important conclusions can be reached from the above: (a) the ideal solvent for crystallization dissolves the solid at high temperature, but not at low temperature, and (b) complete recovery of the solid is impossible by this method. A certain amount of solid will always remain in solution after crystals have formed. A second crystallization can be induced to recover a second crop of solid, but it will be less pure. A typical sequence of steps followed in crystallization experiments is given on page 659, and a list of solvents used in crystallization, arranged by boiling point, is shown on p. 662.