# What principle states that mass is neither gained or lost during a chemical reaction?

$\text{The principle of conservation of mass!}$
$\text{Conservation of mass!}$ is a fundamental, underlying principle of chemistry. In every chemical reaction that has ever been performed, mass is conserved. What does this mean? It means that if you start with 10 g of reactant from all sources, AT MOST you can get 10 g of product. And in practice, you are not even going to get that.
This illustrates the principle of stoichiometry:$\text{garbage in equals garbage out} .$