How do you choose a solvent for recrystallisation?

A solvent for recrystallisation is chosen based on its ability to dissolve the substance at high temperatures but not at room temperature.

Choosing the right solvent for recrystallisation is a crucial step in the purification process of a substance. The ideal solvent should have certain characteristics to ensure the successful recrystallisation of the substance. Firstly, the solvent should be able to dissolve the substance when it is heated, but not when it is at room temperature. This is because the process of recrystallisation involves dissolving the substance in the solvent at a high temperature and then allowing it to cool and recrystallise.

Secondly, the solvent should not react with the substance being purified. This is to ensure that the substance remains unchanged during the recrystallisation process. If the solvent reacts with the substance, it could alter its properties or form a new compound, which would defeat the purpose of the purification process.

Thirdly, the solvent should be volatile, meaning it should easily evaporate. This is because after the substance has recrystallised, the solvent needs to be removed. If the solvent is volatile, it can be easily evaporated off, leaving behind the purified substance.

Lastly, the solvent should not dissolve impurities present in the substance at room temperature. This is to ensure that the impurities are not carried over during the recrystallisation process. If the solvent dissolves the impurities, they will be included in the recrystallised substance, which would affect its purity.

In summary, the choice of solvent for recrystallisation is based on its ability to dissolve the substance at high temperatures but not at room temperature, its non-reactivity with the substance, its volatility, and its inability to dissolve impurities at room temperature.