Heat provides the energy needed to change a solid into a liquid. Heat energy is added to break the bonds between molecules in a solid. The amount of heat needed to cause a solid to turn to a liquid is called the melting point.
By removing heat energy, a liquid can be turned back into a solid. This is called freezing. Generally, matter cycles between the liquid and solid state by the removal or addition of heat energy. Intense heating of a liquid causes vaporization and creates a gas. Solids can skip the melting phase and turn into a gas by a process known as sublimation.