Conservation of mass
scientific law that a closed system's mass remains constant
In physics and chemistry, the law of conservation of mass or principle of mass conservation states that for any system which is closed to all incoming and outgoing transfers of matter, the mass of the system must remain constant 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. 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.
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 finally confirmed 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.
In general, mass is not conserved. The conservation of mass is a law that holds only in the classical limit. For example, the overlap of the electron and positron wave functions, where the interacting particles are nearly at rest, will proceed to annihilate via electromagnetic interaction.
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Datasets, specimens, observations and peer-reviewed methods provide the appropriate test for the technical claims summarized here. The source revision retrieved here is dated Sep 9, 2026. The linked authority identifier is Q483948. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1630.
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This entry incorporates text from “Conservation of mass” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.