Relative Lowering of Vapour Pressure
The fractional reduction in a solvent's vapour pressure caused by dissolving a non-volatile solute.
Formula / equation
(p° − p) / p° = x_solute
Unit
Dimensionless
In depth
Solute particles occupy part of the surface and lower the solvent's escaping tendency, so fewer molecules enter the vapour phase. The relative lowering equals the mole fraction of solute, giving a direct route to molar mass determination from a vapour pressure measurement.
Examples in the real world
Adding sugar to water measurably reduces its vapour pressure, which is why syrup evaporates more slowly than water.