The law of conservation of mass states that the mass of a system of substances is constant, regardless of the processes acting inside the system. An equivalent statement is that matter changes form, but cannot be created or destroyed. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products.
The law of conservation of mass fails for nuclear processes, where the equivalence of matter and energy, and hence conservation of energy, applies. The law also fails for relativistic situations.
The law was first clearly and unambiguously formulated by Antoine Lavoisier, who is often referred to as the father of modern chemistry. However, other scientists such as Mikhail Lomonosov had previously expressed similar ideas.
Conservation of mass finds application as an approximation in cases where relativistic corrections are small, for example in chemistry, particularly in stoichiometric calculations.
See also: conservation law, conservation of energy