Science is full of principles of conservation: conservation of mass, conservation of energy, conservation of charge, conservation of symmetry or "parity," and others. These principles all are statements that, when physical and chemical changes take place, certain properties do not change. Throughout the first part of this book we have been using a conservation principle, although we have not spelled it out explicitly: In chemical reactions matter is neither created nor destroyed, within the limits of our ability to measure mass. The amount of material that comes out of any chemical process is no more and no less than the amount that went in, although the appearance of the material may be changed greatly. In the illustration opposite, propane and oxygen gases react to produce another gas and a liquid. The substances produced look and behave differently, but the total number of atoms of each type is unchanged in the course of the reaction.

Energy also is conserved in chemical reactions, within the limits of our ability to measure it. The amount of energy in the universe at the end of the propane reaction is the same as at the beginning. If a process gives off energy (the propane reaction does), then the product molecules must have less energy than the reactants, by the amount given off.