Fact Check: The Law of Conservation of Mass

In physics and chemistry, 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 the system's mass cannot change, so the quantity can neither be added nor removed. Therefore, the quantity of mass is conserved over time.

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. (E.g., in the Combustion reaction of methane, where 1 atom of Carbon and 4 atoms Hydrogen added to 4 atoms of Oxygen will yield to 1 atom of Carbon and 2 atoms of Oxygen plus 4 atoms of Hydrogen and 2 atoms of Oxygen). Thus, during any chemical reaction and low-energy thermodynamic processes in isolated system, the total mass of the reactants, or starting materials, must be equal to the mass of the products (this is reflected in mathematical form by balancing the chemical equation).

The concept of mass conservation is widely used in many fields such as chemistry, mechanics, and fluid dynamics. Historically, mass conservation in chemical reactions was primarily demonstrated by Jean Rey (in 1630) and later rediscovered by Antoine Lavoisier in the late 18th century. The formulation of this law was of crucial importance in the progress of alchemy to the modern natural science of chemistry.

In reality, the conservation of mass only holds approximately and is considered part of a series of assumptions in 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.

Mass is also not generally conserved in open systems. Such is the case when various forms of energy and matterare allowed into, or out of, the system. However, unless radioactivity or nuclear reactions are involved, the amount of energy escaping (or entering) such systems as heat, mechanical work, or electromagnetic radiation is usually too small to be measured as a decrease (or increase) in the mass of the system.

For systems that include large gravitational fields, general relativity has to be taken into account; thus, mass-energy conservation is as strictly and simply conserved as is the case in special relativity.

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