What law states that mass is neither created nor destroyed in a chemical reaction?

The Law of Conservation of Mass states that mass is neither created nor destroyed during a chemical reaction. This fundamental principle, formulated by Antoine Lavoisier in the late 18th century, highlights that the total mass of reactants before the reaction must equal the total mass of the products after the reaction. In other words, during a chemical process, atoms are rearranged, but the number of atoms, and thus the mass, remains constant.

This law is crucial in chemical equations, where the total mass of the substances involved must be accounted for. It means that if you start with a certain amount of reactants, you will end up with an equivalent amount of products when the reaction is complete, provided that the system is closed and no mass is lost or gained externally.

The other laws mentioned, such as the Law of Definite Proportions and the Law of Multiple Proportions, relate to the ratios of elements in compounds and their combinations, rather than mass conservation specifically. The Law of Conservation of Energy, while also fundamental, deals with energy transformations rather than mass. Thus, the Law of Conservation of Mass is the appropriate law that directly addresses the relationship of mass during chemical reactions.