The Law of Conservation of Mass, proposed by Antoine Lavoisier, states that mass is neither created nor destroyed in a chemical reaction. This means that the total mass of the substances before a reaction (reactants) is equal to the total mass of the substances after the reaction (products). During chemical changes, atoms are simply rearranged to form new substances, but the number and type of atoms remain the same. This law is important in chemistry because it helps scientists balance chemical equations and accurately calculate the amounts of substances involved in reactions.