A common laboratory purification technique is recrystallization. To begin, dissolve a solid compound in a minimum quantity of a hot solvent. On cooling, crystals of the original molecule form. The ideal solvent is one that does not dissolve the solute very well at low temperatures, but dissolves it readily at its boiling point. In general, the best solvent has a slightly lower polarity than the solute and thus the intermolecular interactions of solute with solvent are weaker than the intermolecular interactions of the solute. At higher temperature the solute becomes more soluble. Another useful characteristic for the solvent is a low boiling point, which makes it easier to remove solvent traces from the purified solute crystals. Finally, a good solvent should be a better solvent for any impurities than it is for the desired compound.
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