Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. When you add a nonvolatile solute to a solvent, the vapor pressure of the solution is lower than that of the pure solvent. This phenomenon, called vapor pressure lowering, is one of the colligative properties and is described by Raoult’s law. Understanding it connects to boiling point elevation, freezing point depression, and osmotic pressure.
Raoult’s Law
Raoult’s law states that the partial vapor pressure of each volatile component in an ideal solution equals the vapor pressure of the pure component multiplied by its mole fraction in the solution: P = Xsolvent x P°. Adding a nonvolatile solute (like sugar) to water reduces the mole fraction of water, which in turn reduces the vapor pressure above the solution. The more solute particles you add, the lower the vapor pressure becomes.
Connection to Other Colligative Properties
Vapor pressure lowering is the root cause of boiling point elevation. A lower vapor pressure means the solution needs to be heated to a higher temperature before its vapor pressure equals atmospheric pressure (the boiling point). Similarly, it connects to freezing point depression through the relationship between vapor pressures of solid and liquid phases. The Freezing and Boiling Point Calculator computes these colligative effects, and the Osmotic Pressure Calculator handles the related phenomenon of osmotic pressure.
Real Losungen
Raoult’s law works best for ideal solutions — those where solute-solvent interactions are similar to solvent-solvent interactions. Losungen of similar organic compounds (like benzene and toluene) behave nearly ideally. Losungen of very different substances (like NaCl and water) show significant deviations because ionic dissociation and ion-dipole interactions create non-ideal behavior. For such solutions, the van’t Hoff factor (i) is used as a correction: the effective mole fraction of solvent depends on how many particles each solute molecule produces.