Conservation of mass
mass of an isolated system will always remain constant, regardless of the processes acting inside the system. The matter cannot be created or destroyed, it only changes form. Basically this means that in a closed system : REACTANTS MASSES = PRODUCTS MASSES. This law is non-relativistic - the true (relativistic) situation is somewhat more complicated. The law was first clearly and unambiguously formulated by Antoine Lavoisier, who is often referred to as the father of modern chemistry, although other scientists, such as Mikhail Lomonosov, expressed similar ideas before.
This law finds application as an approximation in cases where relativistic corrections are small - for example in chemistry.
See also conservation law, conservation of energy
