Conservation Of Mass
The law of conservation of mass, also known as the principle of mass/matter conservation, states that the mass of an isolated system (closed to all transfers of matter and energy) will remain constant over time. This principle is equivalent to the conservation of energy: when energy or mass is enclosed in a system and none is allowed in or out, its quantity cannot otherwise change over time (hence, its quantity is "conserved" over time). The mass of an isolated system cannot be changed as a result of processes acting inside the system. The law implies that mass can neither be created nor destroyed, although it may be rearranged in space and changed into different types of particles; and that for any chemical process in an isolated system, the mass of the reactants must equal the mass of the products.
The concepts of both matter and mass conservation are widely used in many fields such as chemistry, mechanics, and fluid dynamics. Historically, the principle of mass conservation, discovered in chemical reactions by Antoine Lavoisier in the late 18th century, was of crucial importance in progressing from alchemy to the modern natural science of chemistry.
In a thermodynamically closed system (i.e. one which is closed to exchanges of matter, but open to small exchanges of non-material energy (such as heat and work) with the surroundings) mass is only approximately conserved. In this case the input or output of energy changes the mass of the system, according to special relativity, although the change is usually small since relatively large amounts of energy are equivalent to only a small amount of mass. Mass is absolutely conserved in so-called isolated systems, i.e. those completely isolated from all exchanges with the environment. In special relativity, the mass-energy equivalence theorem states that mass conservation is equivalent to total energy conservation, which is the first law of thermodynamics. In special relativity the difference between closed and isolated systems becomes important, since conservation of mass is strictly and perfectly upheld only for isolated systems. In special relativity, mass is not converted to energy, as such, since energy always retains its equivalent amount of mass within any isolated system. However, certain types of matter may be converted to energy, so long as the mass of the system is unchanged in the process. When this energy is removed from systems, they lose mass.
In general relativity, mass (and energy) conservation in expanding volumes of space is a complex concept, subject to different definitions, 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.
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