The law of conservation of mass says that in any ordinary chemical reaction, the mass of the reacting substance is exactly equal to the mass of the products. It was stated in 1756 by Mikhail Lomonosov (Russian), and independently by Antoine-Laurent de Lavoisier (French) in 1774. Landolt (German chemist) confirmed it by careful experiments from 1893 to 1908.
Note that the law says "ordinary chemical reactions," excluding nuclear reactions involving radioactive changes. Einstein's theory of mass-energy equivalence states that the mass plus the energy of the reactants in a reaction must equal the mass plus the energy of the products of the reaction. If the mass of the products is less than that of the starting materials, some of the original mass has been converted to energy. This is the process which releases energy when an atomic bomb explodes.
