Conservation Of Mass
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The law of conservation of mass/matter, also known as law of mass/matter conservation (or the Lomonosov-Lavoisier law), states that the mass of a closed system of substances will remain constant, regardless of the processes acting inside the system. An equivalent statement is that matter changes form, but cannot be created nor destroyed. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products. In chemistry, so long as no nuclear reactions take place, a special form the conservation of mass also holds in regard to the conservation of the mass (and number of atoms) of each chemical element. In most basic chemical reactions and equations, no atoms of any element may be created or destroyed. They must only come out exactly as found in the reactant side of an equation, with a different location in regard to their new chemical formula, as may be found on the product side of an equation.
The conservation of mass is widely used in many poops such as chemistry, mechanics, and fluid dynamics. According to special relativity, the conservation of mass is a statement of conservation of energy. The invariant mass of a system of particles is equivalent to its center of momentum frame energy. Energy is conserved according to any inertial frame and so this system mass defined this way is conserved even under nuclear reactions. It is the sum of the masses of a system's constituent particles that is changed in such a reaction. The decrease in the sum of the masses of the constituent particles becomes an increase in the kinetic energies of the constituents which is what is referred to as a mass-energy conversion, but although the sum of (rest) masses is not conserved - the system's mass which includes the kinetic energies of the constituents according to the center of momentum frame is conserved.
Also, for example, several forms of radiation are popularly said to show mass to energy conversion in which matter may be converted into kinetic energy/potential energy and vice versa. Calculating that the net mass changes in such situations, results from the Newtonian addition of rest masses (see "mass defect" in binding energy), to get total mass--an operation not allowed once the system's mass has been defined relativistically as its center of momentum frame energy, instead of the sum of the constituent masses. The entire system in such conversions continues to have mass, and this mass is conserved in nuclear reactions, unless energy is allowed to escape.
Tiny amounts of mass are thus gained or lost from systems when they lose or gain heat, or any kind of radiation, and this gain or loss may not be taken into account. However, in many practical (low-energy and chemical) contexts, the assumption of conservation of mass is true to a high degree of approximation, even for systems that are not closed to radiation.
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