Law of Conservation of Mass
The principle that the total mass of the substances in a closed system is unchanged by any chemical reaction.
Formula / equation
Σ m(reactants) = Σ m(products)
Unit
kg
In depth
Established by Lavoisier through careful weighing of sealed vessels, the law follows from the fact that reactions rearrange atoms rather than create or destroy them. It is the entire justification for balancing chemical equations. In nuclear reactions the law holds only when the mass–energy equivalence of the mass defect is included.
Examples in the real world
Burning 12 g of carbon in 32 g of oxygen produces exactly 44 g of carbon dioxide, no more and no less.