vapor pressure

Meaning: Molecules at a liquid's surface escape into the gas phase while gas molecules return. In a closed container the two rates become equal, and the partial pressure of the vapor at that point is the vapor pressure. A higher temperature gives more molecules enough kinetic energy to escape, so vapor pressure rises steeply. Weaker intermolecular forces (diethyl ether compared with water) give a higher vapor pressure and a more volatile liquid. Adding a nonvolatile solute lowers the solvent's vapor pressure.

Equation: Psolution = Xsolvent × P°solvent, where Psolution is the vapor pressure of the solution (Pa), Xsolvent is the mole fraction of solvent (unitless), and P°solvent is the vapor pressure of the pure solvent at the same temperature (Pa). This is Raoult's law for an ideal solution with a nonvolatile solute.

On the MCAT: Expect vapor pressure versus temperature curves, phase diagrams, and ranking liquids by intermolecular forces. A liquid boils when its vapor pressure equals the external pressure, so lower external pressure (high altitude) lowers the boiling point. At a given temperature, a higher vapor pressure means a lower boiling point. Colligative questions pair a lower vapor pressure with a higher boiling point.

Common confusion: Students think more liquid or a larger container changes the vapor pressure. At a fixed temperature it is set by the liquid's identity, not by the amount of liquid or the container volume, as long as some liquid remains to keep the equilibrium.