Properties of chemical materials are generally classified as either chemical or physical. Chemical properties are associated with any characteristic associated with changing the material's identity; physical properties can be directly measured and often describe a material's state. For solutions, there exists a third class of properties known as colligative properties. These properties are solely dependent on the amount of solute in a given amount of solvent, and are unaffected by the identity of the solute. Colligative properties include freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure.
Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature. It is a measure of the tendency of a material to escape into the environment via vapor, and is an indication of a liquid's evaporation rate. A substance that evaporates quickly has high vapor pressure and is referred to as a volatile substance. Introduction of solute to the solvent results in the lowering of the vapor pressure.
The effect of the presence of solute molecules on the vapor pressure can be understood from a free energy perspective. The tendency of molecules to escape into the gas phase is dependent on how much entropy can be gained by doing so. Escape into the gaseous phase of liquid molecules leads to a increase in entropy due to the greater volume occupied by the molecules in a gaseous state. If the liquid solvent becomes "diluted" with solute, the beginning entropy of the system is already larger, and the amount of entropy to be gained by having molecules escape into the gas phase will be lower. Therefore, one observes a decrease in vapor pressure. Imagine that, for a pure solvent, all the molecules that will escape into the gas phase are situated at the top of the liquid. Introduction of a solute to the solvent results in some of those top surface positions being occupied by solute instead of solvent molecules. Therefore, less solvent will leave the solution into the gas phase, correlating to a decrease in vapor pressure. The amount of solute is directly related to the amount that the vapor pressure is lowered by.(Figure 1)
Raoult's Law states that the vapor pressure of an ideal solution is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution. It is represented by:
Each component's individual vapor pressure is denoted by: