# What happens to mass during chemical and physical changes?

In every chemical and (non-nuclear) physical process EVER observed, mass is CONSERVED. That is, if you start with $10 \cdot g$ of reactant, from all sources, AT MOST you can get $10 \cdot g$ of product. In practice you are not even going to get that, because losses always occur on handling, and every step in a synthesis will remove a proportion of product.