The lowering of the vapor pressure of water due to the addition of solutes, such as acetone or ethanol, is an example of vapor pressure lowering. Vapor pressure is a colligative property of a solution that depends on solute concentration.
Dissolving a solute in a solvent changes the vapor pressure of the solution. The amount of vapor pressure change depends on the interaction between the solvent and solute molecules. Volatile solutes increase the vapor pressure of their solvents. A quantitative formula called Raoult’s law formalizes the relationship between a non-volatile solute and its respective solvent. It states that the vapor pressure of a solution containing a non-volatile solute is directly proportional to the molar fraction of the solute in the solution. As the concentration of the solute in the solution increases, so does the vapor pressure of the solution relative to the vapor pressure of the solvent.
Brine has a higher vapor pressure than pure water for this reason. The relation between the vapor pressure of a solution containing a volatile solute and its solvent is slightly more complex. The vapor pressure of a solution containing one or more volatile solutes is equal to the sums of the products of the molar fractions of each solution component multiplied by the vapor pressure of this component.