What criteria must a mixture meet in order to be separated by fractional distillation?

Answer

Hi Ellen,

In order to be able to separate a mixture of two liquids by fractional distillation, a few conditions must indeed be met. First of all, both boiling points must be different. The smaller the boiling point, the more difficult the distillation. Fractionation is not even necessary with a very large boiling point difference. Then a single distillation is sufficient.

The second condition is a bit more difficult to explain: both liquids must not form an azeotrope. An azeotrope cannot be separated into its pure components by distillation because at the boiling point of the azeotrope the vapor has the same composition as the liquid.

For example, alcohol (ethanol) forms an azeotrope with water with a composition of 95% ethanol and 5% water. So if you distill a mixture of water and alcohol, it is impossible to get more than 95% ethanol. If you still want more pure alcohol, you cannot use distillation, but then you have to let the remaining water in the mixture settle with a water-removing agent. Calcium oxide CaO is sometimes used for this.

The reason you need to heat slowly is because fractional distillation is an equilibrium setting, where vapor and liquid must equilibrate with each other. Achieving that equilibrium takes time, so you should not distill too quickly, and therefore heat slowly.