Heat cause excitation of electrons and molecular vibration. More heat is the more molecular vibration and at the certain point when the energy in molecular vibration exceed the bonding energy, the molecules or atom then set free from the solid bonding and move freely. In absent of atmospheric pressure, the solid would become gas but with the pressure or presence of other gases, the matter may be mobile yet held together in liquid state. The liquid, with more heat, the energy from molecular excitation would set such liquid to become gas.