Raoult's Law

Molecules can only evaporate from the liquid's surface, and dissolving a non-volatile solute means some of that surface is occupied by particles that cannot leave. François-Marie Raoult found in the 1880s that the effect is exactly proportional: the solvent's vapor pressure drops to its mole fraction times the pure-solvent value. Pure water at 25 °C exerts 3.17 kPa; a solution where water's mole fraction is 0.90 (say, 1 mol of glucose per 9 mol of water) exerts P = 0.90 × 3.17 = 2.85 kPa.

This vapor-pressure lowering is the root of all colligative properties — boiling-point elevation and freezing-point depression both follow from it. Real solutions obey Raoult's law best when dilute and when solute–solvent interactions resemble solvent–solvent ones; strong deviations from the straight line are the chemist's first diagnostic that a mixture is far from ideal, like the ethanol–water pair that refuses to distill past 95%.