For example, much of the mass of the average piece of wood is made of polymerized sugars such as cellulose. We can model these chemicals' burning behavior fairly well with glucose, a 6 carbon sugar. When glucose is burned (combined with oxygen) the chemical equation is:
C6H12O6 + 6 O2 -----> 6 CO2 + 6 H2O
glucose + oxygen yields carbon dioxide + water
Add up the carbon, hydrogen, and oxygen atoms on the two sides of the equation and you will see that the numbers are the same. The atoms simply "switch partners" in a form of atomic square dance, but none leave the dance floor.
But what about those processes we agreed to ignore? As the wood burns, chemical potential energy becomes kinetic energy like heat which can presumably radiate out of the fixed area of space you defined in your question. Since Einstein we have known that energy and matter are two forms of the same thing, as steam and ice are two forms of water. So if energy is lost from the system a tiny amount of mass would indeed be escaping. (The amount is tiny as evident from the famous equation E = mc2 where E is energy, m is mass and c is the speed of light in a vacuum, 3 x 108 m/s) Thus we now usually call this law the Law of Conservation of Matter and Energy.
Answered by: Rob Landolfi, Science Teacher