Why is the vapor above a liquid mixture richer in the lower-boiling substance?

In a liquid mixture, the vapor above the surface is richer in the lower-boiling substance because of the relationship between vapor pressure and boiling point.

At any given temperature, each component in a mixture contributes to the total vapor pressure based on its individual tendency to transition into the gaseous state. A substance with a lower boiling point has a higher vapor pressure at a specific temperature compared to a substance with a higher boiling point.12

According to Raoult’s law, the partial pressure of each component in the vapor phase is proportional to its concentration in the liquid phase. Because the more volatile component (the one with the lower boiling point) exerts a higher partial pressure, it constitutes a larger proportion of the molecules in the vapor phase than it does in the liquid phase. This difference in composition between the liquid and vapor phases is the fundamental principle that allows for the separation of substances through distillation.31