Phase change is the process by which a substance changes from a state to another state. For example fusion is phase transfer of a substance from solid to liquid. Similar vaporization is process of changing a gas to a liquid.
The phase change of water is used for better understanding this law.If we discuss water phase change, it is graph between pressure and temperature. Each point in graph shows an equilibrium position. At this position, substance exists in two states at a time. Like the line at which ice and water are separated, the melting and freezing both exits.
Freezing is the process of liquid to solid phase change.If we explain water phase change, we can see that the process starts from solid form of water ice and it is at 0ºC. As temperature increasing upto 75ºC, all the solid state becomes converts into liquid state or ice changes into liquid water.The second line is also called heating curve at which more energy is given to the liquid watre so that it starts to boil. This is done at 100ºC.
The pressure remains constant or it can be upto 1 atm.This is point (d) in diagram. After this point, line (d) to (e) shows the mixture of liquid and steam.All this changes is not due to internal energy of water molecule. We cannot measure it by temperature. This is due to phase change energy of substance. In phase change, the energy comes from potential energy of substance which is related to the molecular attraction.
The potential energy of water is negative. All the added heat energy is used to make the molecular attraction weak. If the energy of phase change is in large amount then this shows the high amount of potential energy which is related to the intermolecular forces of the water molecules. During the melting and vaporization, the energy is used to break down of the solid and liquid bonds and reduces the binding potential energy by changing energy of the phase change.
This does not alter the average value of translational kinetic energy. Sublimation is the process of changing solid phase into gas phase. But solid state changes into gas state without converting into liquid state. This is endothermic process. It requires temperatures and pressures that are lower than the triple point temperature of phase diagram of particular substance. Sublimation example is solid carbon dioxide. Sublimation of carbon dioxide occurs at 197.5 K that is below its triple point temperature 216.6K. Similar iodine is changed into purple vapor from a dark solid that is condensed into a crystalline solid.