Physics heat

When a subatance changes from liquid state to solid state, the potnetial energy should decrease, NOT increase, since the molecules in a solid are closer togetger than in liquid.
Conversely, when a subatance changes from solid to liquid, the potential energy increases because of larger separation between molecules. The energy needed to increase the potential energy comes from the latent heat of fusion.

decrease. because the energy changes to kinetic energy of particles while heat is absorbed.
conservation of energy.
when there is more KE, there is less PE. vise versa.