The law of conservation of mass states that the total mass present before a chemical reaction is the same as the total mass present after the chemical reaction; in other words, mass is conserved.

What happens to matter when it undergoes chemical changes? The Law of conservation of mass says that "Atoms are neither created, nor distroyed, during any chemical reaction." Thus, the same collection of atoms is present after a reaction as before the reaction. The changes that occur during a reaction just involve the rearrangement of atoms.

C7H16(l)+O2(g)→CO2(g)+H2O(g)(3.1.4)(3.1.4)C7H16(l)+O2(g)→CO2(g)+H2O(g) C_7H_{16} (l) + O_2 (g) \rightarrow CO_2 (g) + H_2O (g) \label{3.1.3} The complete combustion of any hydrocarbon with sufficient oxygen always yields carbon dioxide and water. Figure 3.1.23.1.2\PageIndex{2}: An Example of a Combustion Reaction. The wax in a candle is a high-molecular-mass hydrocarbon, which produces gaseous carbon dioxide and water vapor in a combustion reaction (see Equation 3.1.43.1.4\ref{3.1.3}). Equation 3.1.43.1.4\ref{3.1.3} is not balanced: the numbers of each type of atom on the reactant side of the equation (7 carbon atoms, 16 hydrogen atoms, and 2 oxygen atoms) is not the same as the numbers of each type of atom on the product side (1 carbon atom, 2 hydrogen atoms, and 3 oxygen atoms). Consequently, the coefficients of the reactants and products must be adjusted to give the same numbers of atoms of each type on both sides of the equation. Because the identities of the reactants and products are fixed, the equation cannot be balanced by changing the subscripts of the reactants or the products. To do so would change the chemical identity of the species being described, as illustrated in Figure 3.1.33.1.3\PageIndex{3}. Figure 3.1.33.1.3\PageIndex{3}: Balancing Equations. You cannot change subscripts in a chemical formula to balance a chemical equation; you can change only the coefficients. Changing subscripts changes the ratios of atoms in the molecule and the resulting chemical properties. For example, water (H2O) and hydrogen peroxide (H2O2) are chemically distinct substances. H2O2 decomposes to H2O and O2 gas when it comes in contact with the metal platinum, whereas no such reaction occurs between water and platinum. The simplest and most generally useful method for balancing chemical equations is “inspection,” better known as trial and error. The following is an efficient approach to balancing a chemical equation using this method.