<P> The law of conservation of mass or principle of mass conservation states that for any system closed to all transfers of matter and energy, the mass of the system must remain constant over time, as system's mass cannot change, so quantity cannot be added nor removed . Hence, the quantity of mass is conserved over time . </P> <P> The law implies that mass can neither be created nor destroyed, although it may be rearranged in space, or the entities associated with it may be changed in form . For example, in chemical reactions, the mass of the chemical components before the reaction is equal to the mass of the components after the reaction . Thus, during any chemical reaction and low - energy thermodynamic processes in an isolated system, the total mass of the reactants, or starting materials, must be equal to the mass of the products . </P> <P> The concept of mass conservation is widely used in many fields such as chemistry, mechanics, and fluid dynamics . Historically, mass conservation was demonstrated in chemical reactions independently by Mikhail Lomonosov and later rediscovered by Antoine Lavoisier in the late 18th century . The formulation of this law was of crucial importance in the progress from alchemy to the modern natural science of chemistry . </P> <P> The conservation of mass only holds approximately and is considered part of a series of assumptions coming from classical mechanics . The law has to be modified to comply with the laws of quantum mechanics and special relativity under the principle of mass - energy equivalence, which states that energy and mass form one conserved quantity . For very energetic systems the conservation of mass - only is shown not to hold, as is the case in nuclear reactions and particle - antiparticle annihilation in particle physics . </P>

Importance of the law of conservation of mass
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