Antoine Lavoisier's law of conservation of mass states that matter cannot be created or destroyed. It can transition from one form to another via physical and chemical changes. However, even when subjected to a form change, the matter remains conserved. It will exist in the same amount before and after the change, i.e., it is neither destroyed nor created. We can also state that the total mass of reactants = the sum of masses of products + reactants that haven't reacted.