The law of conservation of matter states that matter cannot be created or destroyed. This is relevant to chemical reactions because it tells us that we must have just as many atoms of each element at the beginning of a reaction as we do at the end of a reaction. In the example above, this is reflected by the fact that we use two molecules of oxygen, or four oxygen atoms from our reactants, to produce one molecule of CO₂ and two molecules of H₂O for a total of four oxygen atoms in our products.