Conservation of mass
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The law of conservation of mass/matter (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 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 conservation of mass is widely used in many fields such as chemistry, mechanics, and fluid dynamics. According to special relativity, the conservation of mass is only approximately true for slow moving objects. However in many practical context, the conservation of mass is assumed to be true.
Historical development and importance
The law of conservation of mass (which is effectively conservation of weight when weights are properly taken) was clearly formulated by Antoine Lavoisier in 1789, who is often for this reason (see below) referred to as the father of modern chemistry. However Mikhail Lomonosov (1748) had previously expressed similar ideas and proved them in experiments. Historically, the conservation of mass and weight was kept obscure for millennia by the buoyant effect of the Earth's atmosphere on quantities of gasses, an effect not understood until the vacuum pump first allowed the effective weighing of gasses using scales. Once understood, conservation of mass was of key importance in changing alchemy to modern chemistry. When scientists realized that substances never disappeared from measurement with the scales (once buoyancy had been accounted for), they could for the first time embark on quantitative studies of the transformations of substances. This in turn led to ideas of chemical elements, and the idea of all chemical processes and transformations (including both fire and metabolism) as simple reactions between invariant amounts/weights of these elements.
Approximate conservation vs. serious violation
Even when energy such as heat or light is allowed to enter a system, or escape it, the law of conservation of mass holds to high approximation in cases where energies are small, and special relativity effects can be neglected. In particular, mass is conserved to a high precision in mechanical processes involving macroscopic objects, even if heat is allowed to enter or escape, because the energy and mass associated with this amount of heat, is small.
Similarly, deviations from the conservation of mass are still negligible in chemical reactions, even if heat conservation is neglected. Even in higher energy chemical reactions, the mass-energy of the reactants is huge in comparison to the energy absorbed, retained, or released when they react. By way of example, a gram of TNT releases 4.16 kJ of energy when exploded. However, the rest-energy of a gram of TNT is 90 TJ, or about 20 billion times as much. This means that even if the products of a TNT explosion were stopped and allowed to cool to the original temperature, they would only lose 1 part in 20 billion in weight. This amount would be very difficult to measure.
Serious violations
Drastic violations of the conservation of mass can occur in systems open to escape of energy, for relativistic processes involving very high speeds or very strong fields, such as nuclear and subnuclear reactions and very large astronomical objects. In these situations, whenever a system loses potential energy, and this energy is allowed to escape the system as radiation or heat, the system also loses the corresponding amount of mass with an appropriate factor of c2. Essentially, this loss in mass is ponderable in such systems, because a very great amount of radiation or heat is involved (enough to have appreciable mass). Even here, however, it must be again emphasized that the loss or gain in mass would not appear if the energy associated with it were not allowed to enter or escape the system.
From the non-system view, large violations of conservation of mass in nuclear reactions occur when the total mass of the reaction products is derived in Newtonian fashion from the sum of their rest masses. Such derivation results either from allowing energy to escape (i.e., measuring products at "rest," or with zero momentum, as a whole system); or else is equivalent to changing observers (i.e., the rest mass of products is calculated by using the value for the mass of each product in its own frame of rest). None of these operations are correct in special relativity, although they result in no great errors in Newtonian mechanics.
See also
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