Raoult's Law — Definition

Definition

Imagine you have a pure liquid, say water, in a closed container. Some of the water molecules at the surface gain enough energy to escape into the gaseous phase, creating what we call vapor. This vapor exerts a pressure, known as its vapor pressure.

At a given temperature, this process reaches a dynamic equilibrium where the rate of evaporation equals the rate of condensation, and the vapor pressure becomes constant. Now, what happens if we add something else to this liquid?

This is where Raoult's Law comes into play.

Raoult's Law essentially tells us how the vapor pressure of a liquid changes when we mix it with another substance to form a solution. It considers two main scenarios:

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When the solute is non-volatile: — A non-volatile solute is one that does not readily evaporate (e.g., sugar, salt). When you dissolve a non-volatile solute in a solvent (like water), the solute particles occupy some space at the surface of the liquid. This reduces the number of solvent molecules available to escape into the vapor phase. Consequently, fewer solvent molecules evaporate, leading to a *lower* vapor pressure above the solution compared to the pure solvent. Raoult's Law quantifies this: the relative lowering of vapor pressure (the difference in vapor pressure divided by the pure solvent's vapor pressure) is equal to the mole fraction of the non-volatile solute in the solution. This means the more solute you add, the more the vapor pressure drops.

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When both components (solute and solvent) are volatile: — In this case, both liquids can evaporate. For example, a mixture of ethanol and water. Raoult's Law states that the partial vapor pressure of *each* component in the solution is directly proportional to its mole fraction in the solution. So, if you have a lot of ethanol in the mixture, its partial vapor pressure will be high, and if you have less, it will be lower. The total vapor pressure above the solution is simply the sum of the partial vapor pressures of all the volatile components, as per Dalton's Law of Partial Pressures. Solutions that perfectly obey Raoult's Law over the entire range of concentrations are called 'ideal solutions'. However, most real solutions show some deviations from Raoult's Law, which gives us insights into the intermolecular forces at play.