In this post, we will describe the change of state (phase change) in terms of molecular behavior.
When solids that do not melt are heated, the thermal energy supplied overcomes some intermolecular forces and therefore increases some molecular potential energies. But most of the thermal energy supplied increases the average kinetic energy of the molecules and this is observed as a rise in temperature.
But at the melting point of the solid, all of any thermal energy supplied is transferred to overcoming intermolecular forces, and the solid’s regular molecular structure is lost as the substance becomes a liquid. Since there is no increase in molecular kinetic energies, the temperature remains constant until all the solid has melted.
Similarly, when thermal energy is supplied to a liquid at its boiling point, all of the energy is transferred to overcome the remaining intermolecular forces as gas is formed. Again, since there is no increase in average molecular kinetic energies, the temperature remains constant until all of the liquid has turned to gas.
Conversely, when gases condense or liquids freeze at fixed temperatures, there are changes in molecular potential energies so that thermal energy is released, but there are no changes in molecular kinetic energies.