The law of conservation of mass means that the atoms of an object can not be created or destroyed , but can be moved around and be changed into different particles . This law says that when a chemical reaction makes two different atoms into a new product , the mass of the reactants ( chemicals before the chemical reaction ) is the same as the mass of the products ( the new compound , product made ) . The law implies that mass can not be created or destroyed , although it may be rearranged in space and changed into different types of particles ; and that for any chemical process in a closed system , the mass of the reactants must equal the mass of the products .