The law of conservation of matter states that matter is neither lost nor gained in traditional chemical reactions; it simply changes form. Thus, if we have a certain number of atoms of an element on the left side of an equation, we have to have the same number on the right side. This implies that mass is also conserved during a chemical reaction. The water reaction, for example:2H2 + O2 2H2O
+
2 * 2.02g + 32.00g = 2 * 18.02g
The total mass of the reactants, 36.04g, is exactly equal to the total mass of the products, 36.04g This holds true for all balanced chemical equations.