<P> The law of conservation of mass or principle of mass conservation states that for any system closed to all transfers of matter and energy, the mass of the system must remain constant over time, as system's mass cannot change, so quantity cannot be added nor removed . Hence, the quantity of mass is conserved over time . </P> <P> The law implies that mass can neither be created nor destroyed, although it may be rearranged in space, or the entities associated with it may be changed in form . For example, in chemical reactions, the mass of the chemical components before the reaction is equal to the mass of the components after the reaction . Thus, during any chemical reaction and low - energy thermodynamic processes in an isolated system, the total mass of the reactants, or starting materials, must be equal to the mass of the products . </P>

When is mass conserved according to the law of conservation of mass