Conservation of Mass

Summary: In a chemical reaction, the total mass of reactants equals the total mass of products. Atoms are rearranged, not created or destroyed. Tags: igcse chemistry definition stoichiometry Created: 2026-07-14 Last Updated: 2026-07-14

The law of conservation of mass is a fundamental principle of chemistry stating that matter cannot be created or destroyed during a chemical reaction; consequently, the total mass of all reactants before the reaction must equal the total mass of all products after the reaction. At the atomic level, this is because chemical reactions involve only the breaking and forming of bonds — atoms are rearranged into new combinations, but no atoms are lost, gained, or transmuted into different elements in the process. This law provides the theoretical foundation for all stoichiometric calculations and is the reason why every chemical equation must be balanced: the number of atoms of each element must be identical on both sides of the arrow. Apparent mass changes observed in some reactions do not violate the law; for example, the mass appears to increase when magnesium burns in an open crucible because oxygen from the air combines with the magnesium (mass gain = mass of oxygen added), and the mass appears to decrease when a carbonate reacts with an acid in an open container because carbon dioxide gas escapes to the atmosphere. First demonstrated experimentally by Antoine Lavoisier in the 1770s through careful quantitative measurements, this law remains one of the most commonly tested concepts in IGCSE chemistry, with exam questions frequently asking students to explain why an observed mass change occurs in a given reaction using the principle that mass is conserved.

Key term — IGCSE Chemistry (0620/0971) glossary.