1. Simple and fractional distillations are the solution to solutions
Have a solution when you want to solve a problem? Great! Have a solution when you want to separate its components? Trouble! A solution is so well-mixed that filtration and separating funnel do not provide an easy way out. Instead, we resort to simple and fractional distillations to separate and purify the liquid, through an arduous process of heating and cooling.
The general process of both distillations is the same. We heat a solution in a round-bottom flask to allow the component with a lower boiling point to vaporise first. The pure vapour then passes into the condenser, where it cools and condenses to form the distillate.
2. Fractional distillation is for a solution of multiple miscible liquids
However, distillation gets tricky when the solution contains two or more miscible liquids. An example is a solution of water and ethanol.
Ethanol and water have highly similar boiling points. Consequently, when ethanol boils first at 78 °C, a substantial amount of water vapour will also be evaporating. This contaminates the ethanol vapour.
To purify the vapour, we need a fractionating column to preferentially condense the water vapour and return it to the round-bottom flask. Only ethanol vapour remains and drifts into the condenser, where it cools and condenses to form the pure ethanol distillate.
Simple distillation separates a liquid from a solid-liquid solution while fractional distillation separates a liquid from a solution of two miscible liquids.
3. Fractionating column: you shall not pass
The fractionating column is so long that heat does not distribute evenly. It is hotter below and colder on top.
At any point along the column, a vapour condenses if the temperature there is below its boiling point. Only when the temperature rises to match the boiling point can the vapour remain a vapour and be allowed to pass.
Therefore, the first distillate will be the liquid with a lower boiling point. Its boiling point can be reached more easily at the very top of the column, allowing it to distil over first.
The liquid with a high boiling point distils later, as the second fraction. As we collect the two liquids in two separate fractions, we call this modified form of distillation fractional distillation.
In fractional distillation, the first distillate is the liquid with the lower boiling point.
4. Monitoring temperature change during fractional distillation
We can monitor the progress of fractional distillation by placing a thermometer at the very top of the column. This will tell us whether a vapour will condense or pass into the condenser.
Let’s examine the distillation of a solution of water and ethanol. From 10 min to 20 min, the temperature hits 78 °C and becomes equal to the boiling point of ethanol. This allows ethanol to remain as a vapour and pass into the condenser for condensation and collection. However, water has a higher boiling point of 100 °C. As the temperature is lower than its boiling point, it condenses back into water droplets, dripping down into the round-bottom flask.
