The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction or physical change. The total mass of the reactants before a reaction is equal to the total mass of the products after the reaction. This means the mass of a closed system remains constant over time regardless of how matter rearranges or changes form during the process.
In chemical reactions, the atoms present in the reactants are rearranged to form the products but the number and types of atoms remain the same, so the total mass is conserved. For example, burning wood reacts with oxygen, and the mass of wood and oxygen consumed equals the mass of carbon dioxide, water vapor, smoke, and ash produced. Similarly, physical changes like melting or evaporating water do not change its mass.
This fundamental principle was first clearly formulated by Antoine Lavoisier in 1789 and serves as a cornerstone of modern chemistry and physics.