According to the law of conservation of mass, "In an isolated system, mass cannot be created or destroyed, but it can be transformed from one form to another."
For a law-energy thermodynamic process, the mass of the reactants must be equal to the mass of the products according to the law of conservation of mass.
It is thought that mass conservation is defined by a few classical mechanics assumptions.
With the help of quantum mechanics and special relativity, the law of conservation of mass was later modified to state that energy and mass are one conserved quantity.
Antoine Laurent Lavoisier discovered the law of conservation of mass in 1789.