Compare evaporation, crystallisation and distillation. In which situation, would you prefer each of these over the others?
Evaporation, crystallisation, and distillation are three different techniques used to separate the components of a mixture, and each one is preferred in a specific condition depending on what we want to recover.
Comparison of the three methods:
| Basis | Evaporation | Crystallisation | Distillation |
|---|---|---|---|
| Principle | A liquid is converted into vapour on heating, leaving behind the dissolved solid. | A pure solid is obtained as crystals from its saturated solution by cooling or slow evaporation. | A liquid is vaporised on heating, and the vapour is cooled and condensed back into a liquid. |
| Used for | Separating a non-volatile solute from a solvent. | Separating and purifying a solid from its solution. | Separating a liquid from a solution or separating two miscible liquids with a boiling point difference of about 25 °C or more. |
| Recovery of solvent | Solvent is lost as vapour. | Solvent mostly remains as mother liquor, though some may evaporate during concentration. | The liquid that vaporises is recovered as distillate; the other component remains behind. |
| Purity of product | The solid obtained may contain impurities. | Gives purer crystals because many impurities remain in the solution. | Gives a relatively pure liquid distillate. |
Evaporation is preferred when we only want to recover the solid solute and the solvent is not required, such as obtaining salt from seawater. Crystallisation is preferred when we want to obtain a pure solid from a solution containing soluble impurities, such as purifying copper sulfate. Distillation is preferred when we want to recover a liquid solvent from a solution or separate two miscible liquids with sufficiently different boiling points, such as separating acetone from water.
