Vapor Pressure

• The molecules at the surface can spontaneously go into a gas as KE increases enough to break attractive forces.

Vapor Pressure and Boiling Point

• Liquid boils when its vapor pressure equals the external pressure (atmospheric pressure) acting on the surface of the liquid.

Quick Demo

Phases

Phase diagrams

• A graph showing the relationships between solid, liquid, gas, and temperature and pressure

• Normal boiling point: the temperature at which a substance boils at 1.00 atm.

Phase Diagram Vocab• Triple Point: temperature and pressure where

solid, liquid, and gas all coexist

• Critical Point: Is it a gas or liquid – who knows?– Critical Temp- highest temp a liquid can form– Critical Press – press required for liquefaction – *** KE of molecules greater than attractive forces!

• Example: Nitrogen CT: 126.1 K, CP: 33.5 atm

Phase DiagramsPhase diagrams display the state of a substance at various pressures and temperatures.

Phase Diagrams• The triple point (T), the point at which all three

states are in equilibrium.

Phase DiagramsIt ends at the critical point (C); above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other.

Phase Diagrams• Below the triple point the substance cannot exist

in the liquid state.• Along the circled line the solid and gas phases are

in equilibrium; the sublimation point at each pressure is along this line.

Phase Diagram of Water

• Note the high critical temperature and critical pressure.– These are due to the

strong van der Waals forces between water molecules.

Phase Diagram of Water

• The slope of the solid-liquid line is negative.– This means that as the

pressure is increased at a temperature just below the melting point, water goes from a solid to a liquid.

Phase Diagram of Carbon Dioxide

Carbon dioxide cannot exist in the liquid state at pressures below 5.11 atm; CO2 sublimes at normal pressures.