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Explanation:
Recrystallization is also known as fractional crystallization. And just like fractional distillation, it is used to seperate a mixture of substances (usually a pure substance and its many impurities).
It uses the theory that solubility of solids in solvents increase with temperature.
In normal crystallization, a single solute is contained in the solution and the solution is heated to a high temperature where all of the solute is soluble and then it is gently cooled to enable the solute form as crystals as this temperature drops.
In fractional crystallization or recrystallization, the solutes are dissolved in a 'suitable solvent' at high temperature or dissolved at low temperature and heated to a temperature where all of the solute become soluble. Then, the mixture is cooled slowly such that the solute of interest (the impure solute) crystallizes out first before the other solutes (which are impurities).
So, the properties of a very good recrystallization solvent that can do this job effectively, includes:
- Probably the most important characteristic is that a good recrystallization solvent does not react with the substance to be purified.
- Then, a good recrystallization solvent shouldn't dissolve the substance to be purified at room temperature, but it should dissolve it well at high temperatures or at the solvent's boiling point.
- A good recrystallization solvent should properly dissolve soluble impurities at room temperature.
- A good recrystallization solvent should not dissolve insoluble impurities even at the solvent's boiling point.
- The solvent should not be costly.
- The solvent should be volatile enough so that it can be easily separated from the purified product.
