Law Of Mass Conservation

The law of conservation of mass, also known as the principle of mass/matter conservation, states that the mass of a closed system (in the sense of a completely isolated system) willremain constant over time. This principal is equivalent to the conservation of energy, in the sense when energy or mass is enclosed in a system and none is allowed in or out, its quantitycannot otherwise change (hence, its quantity is "conserved"). The mass of an isolated system cannot be changed as a result of processes acting inside the system. The law implies that masscannot be created or destroyed, although it may be rearranged in space and changed into different types of particles; and that for any chemical process in a closed system, the mass ofthe reactants must equal the mass of the products.
The concepts of both matter and mass conservation is widely used in many fields such as chemistry, mechanics, and fluid dynamics.Historically, the principle of mass conservation, discovered by Antoine Lavoisier in the late 18th century, was of crucial importance in changing alchemy into the modern natural science ofchemistry.
In special relativity, the mass-energy equivalence theorem states that mass conservation is equivalent to energy conservation, which is the first law of thermodynamics. Inspecial relativity, mass cannot be converted to energy, since energy always retains its equivalent amount of mass. However, certain types of matter may be converted to energy, so long as themass of the system is unchanged in the process.
In general relativity, mass (and energy) conservation in expanding volumes of space becomes a complicated concept, subject to differentdefinitions, and neither mass nor energy is as strictly and simply conserved as is the case in special relativity and in Minkowski space. For a discussion, see mass in general relativity.