Vapor Pressure• Pressure exerted by a vapor in equilibrium with

its liquid or solid phase

Vapor Pressure• At any temperature, some molecules in a

liquid have enough energy to escape.• As the temperature rises, the fraction of

molecules that have enough energy to escape increases.

Vapor Pressure

As more molecules escape the liquid, the pressure they exert increases.

Vapor Pressure

The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate.

Gas-Liquid Equilibration

Liquid Evaporates when no Equilibrium is Established

Vapor Pressure• The boiling point of a

liquid is the temperature at which its vapor pressure equals atmospheric pressure.

• The normal boiling point is the temperature at which its vapor pressure is 760 torr.

Vapor Pressure and Boiling PointVapor Pressure and Boiling Point•Liquids boil when the external pressure equals the vapor pressure.•Temperature of boiling point increases as pressure increases.•Two ways to get a liquid to boil: increase temperature or decrease pressure.–Pressure cookers operate at high pressure. At high pressure the boiling point of water is higher than at 1 atm. Therefore, there is a higher temperature at which the food is cooked, reducing the cooking time required.

•Normal boiling point is the boiling point at 760 mmHg (1 atm).

Phase DiagramsPhase Diagrams• Any temperature and pressure combination not

on a curve represents a single phase.

Clausius-Clapeyron Equation