Why is energy needed to change a solid into a liquid???

To change a solid into a liquid, or a liquid into a gas, requires heat energy. This heat energy allows the change of state to happen, and the temperature remains constant during the process.

the product of the pressure and volume is also constant

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.