In all chemical changes, whether they are reversible or not, the same number of atoms or is present before and after the changes. For example, the equation for forming carbon dioxide is C + o2-> co,. One atom of carbon and two atoms of oxygen are on each side of the equation. In other words, the total mass of the reactants is the same as the total mass of the products. Because the mass stays the same, it is said to be conserved. In physical changes, the basic chemical nature of matter is not changed. The properties of a substance, such as its shape, size, or physical state, are altered. However, the identity of the substance does not change.