In Chapter 11 we saw two examples of principles of conservation, involving matter and charge, in chemical reactions. Neither the total mass nor the net oxidation number of reactants and products can change during a chemical transformation. Mass and oxidation number are more fundamental properties of substances than are volume, color, texture, density, or electrical conductivity.

A third important property that is conserved is energy. Combustions and many other reactions give off energy in the form of heat, and this makes these processes useful to us. But if we draw an imaginary box around the reacting substances, large enough to contain the substances and everything else they interact with, then the total energy within that box will not change during the reaction. (This is one form of the first law of thermodynamics.)

If the reaction gives off heat energy, then the products must have less energy than the reactants, because this difference is the only source of the heat. Conversely, if the products have more energy than the reactants, this extra energy must be supplied to the reaction from outside.