Reality → Matter → Molecules → Stoichiometry
In chemical reactions, conservation of mass is one of the most fundamental and earliest laws of chemistry, first formulated by Lavoisier [1] . Also, a reaction product always is made up of exactly the same proportion of its constituent elements. This is a second fundamental law of chemistry, first observed by Proust, then used by Dalton to formulate the first scientifically based atomic theory. Both laws, together with the notion of valence, form the base for practical stoichiometry, used by chemists to write equations and formula for chemical reactions, and by engineers to calculate the mass balances (or input-output streams) of chemical processes in industrial plants.
After Einstein's formulation of special relativity and the introduction of quantum physics, mass conservation is no longer a universally true law. In nuclear reactors, the fission products show a small mass deficit, providing a proof for mass-energy equivalence. In chemical reactions the mass change is unmeasurably small, but theoretically also exists.