Vapour Pressure of Liquid Solutions — Definition

Definition

Imagine you have a pure liquid, say water, in a closed container at a specific temperature. The molecules at the surface of the liquid are constantly escaping into the space above the liquid, forming a gas (vapour).

At the same time, some of these vapour molecules are losing energy and returning to the liquid phase. Eventually, a state of dynamic equilibrium is reached where the rate at which liquid molecules turn into vapour is exactly equal to the rate at which vapour molecules turn back into liquid.

The pressure exerted by these vapour molecules on the walls of the container, at this equilibrium, is called the vapour pressure of the pure liquid. This pressure is characteristic of the liquid and depends only on temperature; higher temperatures mean more molecules have enough energy to escape, leading to higher vapour pressure.

Now, let's consider a liquid solution. A solution is a homogeneous mixture of two or more substances. When we talk about the vapour pressure of a liquid solution, we're interested in how the presence of a solute affects this pressure. There are two main scenarios:

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Solution with a Non-Volatile Solute — If you dissolve a substance that does not readily vaporize (a non-volatile solute, like sugar or salt) into a volatile solvent (like water), something interesting happens. The solute particles occupy some of the surface area of the solvent. This means fewer solvent molecules are exposed at the surface, and thus, fewer solvent molecules can escape into the vapour phase per unit time. Consequently, the rate of evaporation of the solvent decreases. Since the rate of condensation remains relatively unchanged initially, the equilibrium is established at a lower vapour pressure compared to the pure solvent. So, adding a non-volatile solute *lowers* the vapour pressure of the solution.

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Solution with a Volatile Solute — If you mix two or more volatile liquids (like ethanol and water), both components will contribute to the total vapour pressure of the solution. Each component will exert its own partial vapour pressure, and the total vapour pressure of the solution will be the sum of these partial pressures. The partial vapour pressure of each component in the solution is directly proportional to its mole fraction in the solution and its vapour pressure in the pure state. This relationship is precisely what Raoult's Law describes. Understanding vapour pressure of liquid solutions is vital because it helps us predict how solutions will behave, especially in processes like distillation, and forms the basis for understanding colligative properties.