The change of a solid into a liquid is called melting. This typically occurs when a solid absorbs heat.
The molecules of the solid start vibrating more vigorously as they absorb heat. The kinetic energy of the molecules increases when the temperature increases.
At the melting point, the molecules gain enough energy to overcome the intermolecular attractive forces. The molecules then begin to break away from their fixed positions and the solid begins to melt.
The temperature at which a solid turns into a liquid is known as the melting point. The melting point of a particular compound at a set pressure is always the same.
This is because the molecules need a fixed amount of energy to overcome the intermolecular forces that hold them together in a solid.