All changes in state are physical changes. These processes are reversible. There is no change in the structure of the atoms or molecules involved. They can go back and forth between states with small inputs or withdrawals of heat. The mixture of ice cubes and liquid water can be 'pushed' to more ice if the mixture is cooled. Conversely the amount of ice can be decreased if a little more heat is added to te mixture. Some people are surprised to see that the ice and water mixture has a constant temperature until either all the ice melts or all the water freezes. Changes in state occur between a condensed form and an expanded or less condensed state. For example boiling a liquid converts a condensed state (the liquid) with particles close together to a dispersed state (the gas). The volume increase from liquid to gas is approximately a factor of 1000. The liquid is more condensed and has much less volume than the gas. This volume increase is one reason why aerosol cans have warnings saying they should not be incinerated. When the liquid in the aerosol can is vaporized the gas pressure will increase enough to explode the thin walled can. This rollover picture and the youtube video show what happens to an aersol can in a fire. Click to see youtube video

  

Changes in state are accompanied by a change in potential energy. This makes sense because the positions of particles in the material are different after the change. Remember potential energy is energy resulting from position. If the positions of the particles in the substance change then the potential energy changes. Changes in state happen at definite transition temperatures under definite pressures. Boiling points are very pressure sensitive. Melting points are not influenced much by changes in pressure. Pressure is more important for boiling points than melting points. The reason for this is that liquids and solids have comparable volumes. When a solid melts the liquid volume is similar.

The common phase or state changes are listed in this table.

Compact state	process	Less compact state	Heat effect type	Name for heat effect
Solid	melting---->	Liquid	endothermic	Heat of fusion
Liquid	boiling ----->	Gas	endothermic	Heat of vaporization
Solid	sublimation--->	Gas	endothermic	Heat of sublimation
Gas	condensation-->	Liquid	exothermic	Heat of condensation
Gas	condensation-->	Solid	exothermic	Heat of condensation
Liquid	freezing ------>	Solid	exothermic	Heat of crystallization