the molecules gain or los energy,the more energy these molecules have the easy it is for them break the bonds holding them together, thus when you apply enough heat to a substance it changes from a solid then liquid from a liquid to gas
When matter absorb heat the intermolecular forces in between the particles decreased. Making them having a high kinetic energy. The particles would be far apart from each other after a several stage of absorbing heat. That is why matter would change its physical state from solid to liquid and liquid to solid. And as for release heat. . it is just vice versa to that of the above statement.
