Chemical reactions involve the rearrangement of atoms and the formation of new compounds with different physical properties than the original reactants. Signs that a chemical reaction has occurred include the formation of a gas, bubbles, an odor, smoke, a new solid, or a change in color. Chemical reactions also involve the absorption or release of energy. Exothermic reactions release energy overall as the products have less stored energy than the reactants. Endothermic reactions absorb energy overall as the products have more stored energy than the reactants.