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 cannot be created nor destroyed, although it may change form. 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 mass/matter conservation may be considered as an approximate physical law that holds only in the classical sense before the advent of special relativity and quantum mechanics.
This historical concept is widely used in many fields such as chemistry, mechanics, and fluid dynamics.
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[edit] Historical development and importance
The law of conservation of mass was dependently 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 the weight of gases, an effect not understood until the vacuum pump first allowed the effective weighing of gases 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, as well as the idea that all chemical processes and transformations (including both fire and metabolism) are simple reactions between invariant amounts/weights of these elements.
[edit] Generalization
In special relativity, the conservation of mass does not apply.
The principle that the mass of a system of particles is equal to the sum of their masses, even though true in classical physics, is false in special relativity. The mass-energy equivalence formula implies that bound systems have a mass less than the sum of their parts. The difference, called a mass defect, is a measure of the binding energy — the strength of the bond holding together the parts (in other words, the energy needed to break them apart). The greater the mass defect, the larger the binding energy. The binding energy is released when the parts combine to form the bound system. [1]
Other violations of the conservation of mass can occur in special relativity. For example, when matter is converted to massless energy according to E = mc². As another example, when an atom emits a photon (which is massless), the atom's mass is decreased by E/c² where E is the photon's energy. The mass of closed (isolated) systems can diminish by emission of photons, even if the photons remain inside the system.[2]
In special relativity, the conservation of mass can also not be cast as a statement of conservation of energy. A system of two photons can be massless or have an inertial mass up to 2E/c², where E is each photon's energy (assumed equal), as a function of relative momentum orientation for the photons. So, independently of the energy content being constant at 2E, the total mass may vary from zero to 2E/c².[3]
The conservation of mass also does not apply to particles created by pair production.