Every chemical reaction involves a change. A chemical reaction occurs when one or more molecules combine with another to form a new chemical. Without that chemical change, the result cannot be called a reaction.
A chemical reaction can be made up of a number of different things, including compounds, atoms, ions and the molecules found within a single element.
Because the process also includes the creation and breaking of covalent bonds, a transfer of energy almost always occurs as well in the form of heat, light, sound, electrical current and the process of producing gases. This is because the presence of energy is required for a bond to properly break.