Vapour Pressure of Solutions of Solids in Liquids — Definition

Definition

Imagine you have a pure liquid, like water, in a closed container. Some of the water molecules at the surface gain enough energy to escape into the air above the liquid, forming a gas (vapour). At the same time, some of these vapour molecules lose energy and return to the liquid state.

Eventually, a balance is reached where the rate of evaporation equals the rate of condensation. The pressure exerted by the vapour at this equilibrium is called the vapour pressure of the pure liquid.

Now, let's consider what happens when we dissolve a solid substance, like sugar or salt, into this water. These solid substances are typically 'non-volatile,' meaning they themselves do not easily turn into a gas at room temperature or even when heated moderately.

When you add such a non-volatile solid to a volatile liquid (like water), it forms a solution. The solute particles (sugar or salt) spread out and occupy some of the space at the surface of the liquid.

Because some of the surface area is now occupied by non-volatile solute particles, fewer solvent molecules (water molecules) are exposed at the surface. This means that fewer solvent molecules can escape into the vapour phase per unit time.

Consequently, the rate of evaporation of the solvent decreases. While the rate of condensation of solvent molecules from the vapour phase remains largely unaffected initially, the reduced evaporation rate leads to a new equilibrium where there are fewer solvent molecules in the vapour phase.

This results in a lower pressure exerted by the vapour above the solution compared to the pure solvent at the same temperature. This reduction in vapour pressure is a direct consequence of the presence of the non-volatile solute.

The extent to which the vapour pressure is lowered depends on the concentration of the solute, specifically the number of solute particles present, rather than their chemical identity. This property is known as a colligative property, a concept we will explore further.

This phenomenon is crucial for understanding many chemical and biological processes, and it forms the basis for other colligative properties like boiling point elevation and freezing point depression.