Vapour Pressure of Liquid Solutions — Core Principles

Core Principles

Vapour pressure is the pressure exerted by the vapour in equilibrium with its liquid phase at a given temperature. For pure liquids, it increases with temperature. When a non-volatile solute is added to a volatile solvent, the vapour pressure of the solution decreases because fewer solvent molecules are available at the surface to vaporize.

Raoult's Law quantifies this, stating that the partial vapour pressure of a volatile component in a solution is proportional to its mole fraction ($P_A = X_A P_A^0$). For solutions with multiple volatile components, the total vapour pressure is the sum of their partial pressures ($P_{total} = X_A P_A^0 + X_B P_B^0$).

Ideal solutions strictly obey Raoult's Law, exhibiting no heat or volume change on mixing, with similar intermolecular forces. Non-ideal solutions deviate: positive deviations show higher vapour pressure (weaker A-B interactions, \Delta H_{mixing} > 0, \Delta V_{mixing} > 0), while negative deviations show lower vapour pressure (stronger A-B interactions, \Delta H_{mixing} < 0, \Delta V_{mixing} < 0).

Significant deviations can lead to azeotropes, constant boiling mixtures that cannot be separated by fractional distillation.

Important Differences

vs Non-Ideal Solutions

Aspect	This Topic	Non-Ideal Solutions
Obedience to Raoult's Law	Obeys Raoult's Law over the entire range of concentration.	Does not obey Raoult's Law over the entire range of concentration.
Intermolecular Forces	A-B interactions are similar to A-A and B-B interactions.	A-B interactions are different from A-A and B-B interactions (either weaker or stronger).
Enthalpy of Mixing (\Delta H_{mixing})	Zero (no heat absorbed or released).	Non-zero (either positive for endothermic or negative for exothermic).
Volume of Mixing (\Delta V_{mixing})	Zero (no change in volume upon mixing).	Non-zero (either positive for expansion or negative for contraction).
Vapour Pressure	Total vapour pressure is intermediate between pure components, varying linearly with mole fraction.	Total vapour pressure is either higher (positive deviation) or lower (negative deviation) than predicted by Raoult's Law.
Examples	Benzene + Toluene, n-Hexane + n-Heptane, Chloroethane + Bromoethane.	Positive deviation: Ethanol + Water, Acetone + Ethanol. Negative deviation: Acetone + Chloroform, Nitric acid + Water.

Ideal solutions represent a theoretical benchmark where components mix without any change in energy or volume, and their vapour pressures strictly follow Raoult's Law due to identical intermolecular forces. In contrast, non-ideal solutions are the norm in reality, exhibiting deviations from Raoult's Law because the intermolecular forces between solute and solvent differ significantly from those within the pure components. These differences lead to measurable heat and volume changes upon mixing, and consequently, the observed vapour pressure is either higher or lower than what Raoult's Law would predict for an ideal mixture.