Conservation of mass

The principle that total mass in a closed system remains constant, even as matter changes form or moves between parts of the system.

Mass balance: An accounting equation equating matter entering a system with matter leaving it and accumulating within it. It applies conservation of mass to systems with defined inputs, outputs, and storage.
Antoine Lavoisier: An eighteenth-century French chemist who helped establish modern quantitative chemistry. His sealed-vessel experiments supplied influential evidence for mass conservation in reactions.
Chemical engineering: The engineering discipline that designs processes for producing and transforming chemicals and materials. Engineers use material balances to size equipment and track industrial process streams.
Mass–energy equivalence: The principle that mass and energy are related by the equation E = mc². It places ordinary mass conservation within the broader conservation of mass-energy.
Chemical equation: A symbolic representation of a chemical reaction showing reactants, products, and their relative amounts. Balanced equations express equal numbers of each kind of atom before and after a reaction.
Joseph Priestley: An English chemist and natural philosopher who studied gases, including oxygen. His gas experiments informed the chemical investigations through which Lavoisier developed the principle.
Reactor design: The design of equipment and operating conditions for carrying out chemical reactions. Reactant and product balances constrain reactor size, feed rates, and expected yields.
Conservation of energy: The principle that total energy in an isolated system remains constant. Unlike mass conservation in classical chemistry, energy conservation remains fundamental in modern physics.
Stoichiometry: The quantitative study of the amounts of reactants and products in chemical reactions. It uses balanced reactions to calculate how conserved matter is apportioned.
Phlogiston theory: An obsolete theory that explained combustion through the release of a substance called phlogiston. Lavoisier's measurements challenged its account of combustion and helped replace it.

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