The law of conservation of mass states that, "Mass can neither be created nor destroyed, only its form changes with respect to different forms of energy."
In any reaction, say 10 gms of water is decomposed into its constituents Hydrogen and Oxygen. But the mass remains conserved for the reaction. Although mass and energy are inter-convertible. But,the net mass/energy remains conserved i.e. the mass of products will be equal to the mass of reactants.
