A chemical change is a process in which one or more substances combine to generate new compounds, with different chemical properties from the starting reagents. Chemical processes are associated with energy exchanges.
An example of a chemical change is the combustion of magnesium. The reaction of magnesium and oxygen forms a new substance, magnesium oxide, whose physical and chemical properties do not resemble either magnesium or oxygen.
In physical changes does not change the chemical composition of substances. The change from solid to liquid water (melting) is a physical change. During this process the physical properties of the water change, but it is not transformed into another substance, it is still water.
Physical changes, like chemicals, involve exchanges of energy. The melting of water is an endothermic process and requires an input of energy.