Phase Diagrams and Vapor Pressure Diagrams

A substance can change phase due to either pressure changes or temperature changes. The intermolecular forces keeping a substance together in its solid state must be overcome in order for the substance to melt into a liquid state. Likewise, the intermolecular forces holding droplets of liquid together must be overcome to vaporize the substance. The means by which these forces are overcome can vary. Lowering the pressure allows more liquid molecules to hit and break through the surface and become a gas. If the pressure is increased, the gas molecules are forced back into the liquid state. Phase changes occur at many different temperatures. Remember, normal melting and boiling point are when the pressure is held to standard pressure.