What does the law of conservation of mass state?

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. This fundamental principle establishes that during any chemical transformation, the total mass of the reactants must equal the total mass of the products. Essentially, the atoms involved in the reaction are rearranged, but their total quantity remains constant throughout the process.

For example, if you start with a certain amount of reactants, after the reaction has occurred, the mass of the products will equal that of the reactants—this underscores that no mass is lost or gained. This principle is crucial in understanding chemical reactions, stoichiometry, and the fundamental behavior of matter in physical science.

The other choices don't align with this principle: the idea that mass can be created during chemical reactions contradicts the law; claiming that mass is transformed into energy relates to Einstein's theory of relativity rather than chemical reactions; and the notion that mass decreases in every chemical reaction is also incorrect, as it misinterprets the zero-sum principle of mass in reactions.