Relative Lowering of Vapour Pressure — Definition
Definition
Imagine you have a pure liquid, like water, in a closed container. Some of the water molecules at the surface have enough energy to escape into the air above the liquid, forming a gas (vapour). This vapour exerts a pressure, which we call vapour pressure.
At a given temperature, there's a specific amount of vapour pressure that pure water will exert. Now, what happens if you dissolve something in this water that doesn't easily turn into a gas itself, like sugar or salt?
We call this a 'non-volatile solute'. When you add sugar to water, the sugar molecules spread out and occupy some of the space at the surface of the water. This means there are now fewer water molecules directly at the surface, compared to when it was pure water.
Since fewer water molecules are at the surface, fewer of them can escape into the vapour phase at any given moment. Consequently, the number of water molecules in the vapour phase above the solution decreases, and so does the pressure they exert.
This reduction in vapour pressure is known as the 'lowering of vapour pressure'. It's like having fewer doors for the water molecules to escape through. The more sugar you add, the more surface area is occupied by sugar molecules, and the lower the water's vapour pressure becomes.
The term 'relative lowering' comes into play when we compare this decrease in vapour pressure not just to zero, but to the original vapour pressure of the pure solvent. Specifically, it's the ratio of the lowering of vapour pressure (the difference between the pure solvent's vapour pressure and the solution's vapour pressure) to the vapour pressure of the pure solvent itself.
This ratio is a very special quantity because it depends only on the number of solute particles, not their identity, making it a 'colligative property'. This means whether you add sugar or urea, if you add the same number of particles, the relative lowering of vapour pressure will be the same.
This property is incredibly useful for determining the molar mass of unknown non-volatile solutes, which is a common application in chemistry.