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  of the mass  system must remain constant over time, as system mass cannot change quantity if it is not added or removed. Hence, the quantity of mass is “conserved” over time. 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, as for example when light or physical work is transformed into particles that contribute the same mass to the system as the light or work had contributed. Thus, during any chemical reaction, nuclear reaction, or radioactive decay in an isolated system, the total mass of the reactants or starting materials must be equal to the mass of the products.

In special relativity, mass is not converted to energy, since mass and energy cannot be destroyed, and energy in all of its forms always retains its equivalent amount of mass throughout any transformation to a different type of energy within a system or translocation into or out of a system. Certain types of matter a different concept may be created or destroyed, but in all of these processes, the energy and mass associated with such matter remains unchanged in quantity although type of energy associated with the matter may change for.