Nss1 Physics question

When a liquid cools down and undergoes a change of state, the released energy makes the separation between molecules in the liquid to become shortened. The potential energy thus reduced. Intermolecular force then holds the molecules more tightly and the molecules are not free to move. A solid is thus formed.

Therefore, the energy lost to the surroundings comes from the reduction of potential energy of the molecules, with the kinetic energy remains unchanged. Thus the temperature becomes constant.