Mass of closed system rules

The law of conservation of mass, also known as the principle of mass/matter conservation, states that the mass of a closed system (in the sense of a completely isolated system) will remain constant over time. This principle is equivalent to the conservation of energy, in the sense when energy or mass is enclosed in a system and none is allowed in or out, its quantity cannot otherwise change (hence, its quantity is "conserved"). The mass of an isolated system cannot be changed as a result of processes acting inside the system. The law implies that mass cannot 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.

Conservation of mass - Wikipedia, the free encyclopedia