Question

For a distillation lab, why wasn’t the first drop the boiling point of the lower boiling substance?

Answer 1

The mixture to be distilled consists of a higher and lower boiling substances. All of these substances have their respective vapor pressures. Also, it can be seen from the various vapor liquid equilibrium curves that at all the temperatures, the vapor always consists of a mixture of substances which is richer in the lower boiling specie than the higher boiling one. Hence, the first drop of liquid in the condenser will be at a slightly higher temperature than boiling point of the lower boiling substance as it is a mixture rather than the pure compound in the vapor state.