When a solid is heated, its kinetic and potential energies increases. Up to a certain point, the kinetic and potential energy is so high that any heating done to it would only increase the potential energy and not the kinetic energy of the solid anymore. When this happens, some of the intermolecular forces are weakened and so the average distance between particles increase (the potential energy increases) while the kinetic energy remains the same. I'm not sure why there is a certain threshold of KE and PE then melting occurs actually. Why is this so? Then after the solid is fully melted and heated, the kinetic energy will increase with the potential energy. This goes on until a certain point where only the potential energy increases (boiling). When boiling occurs, now the intermolecular forces are completely broken and so the average distance between particles continue to increase (PE increases) while the kinetic energy remains the same. Again I'm not sure what determined the threshold of KE and PE to which boiling occurs. Why don't the KE continue increasing with the PE until the bonds are weakened such that a liquid is forms. In other words, why can't it be a gentle increase in both energies so that the change from one state to another isn't indicated by the constant KE and increasing PE and instead the gradual change from solid to liquid cannot be pinpointed at one specific level of KE and PE. Could anyone explain why must the KE be constant during any of this processes?