Law of Conservation of Mass

The law of conservation of mass states that the total mass of a closed system remains constant over time, regardless of the physical or chemical transformations taking place within it, so matter is neither created nor destroyed in an ordinary chemical reaction. French chemist Antoine Lavoisier is generally credited with establishing the law through careful, quantitative combustion experiments in the 1770s, work that helped overturn the phlogiston theory then dominant in chemistry and laid groundwork for modern chemistry as a quantitative science. The law holds as an excellent approximation for chemical processes, though Einstein's mass-energy equivalence later showed that mass and energy are interconvertible in nuclear reactions, where a small measurable mass defect corresponds to the energy released.