During any chemical reaction we do not lose atoms, we do not gain atoms. What we do in fact is rearrange atoms from original reactants to new products. We cannot lose or gain atoms and this leads us to the law of conservation of mass in chemical reactions which states:


"During  a chemical reaction atoms are not made or destroyed, they are simply rearranged so the mass of the end products equals the mass of the starting reactants"


This does make the arithmetic a little bit simpler because we know that we can't lose or gain anything.


This can be shown quite simply by the complete combustion of a mole of methane gas in oxygen:



If we look at the left-hand side of the equation, we can see that in methane we have one carbon atom and 4 hydrogen atoms and in total we have 4 oxygen atoms. Now look at the right hand side, we have one carbon atom, 4 oxygen atoms and 4 hydrogen atoms, in other words we have ended up with what we started with but rearranged differently as carbon dioxide and water are completely different substances to methane and oxygen. We have lost no matter, we have gained no matter but we ended up with 2 completely new substances.


If we consider molar quantities, we can see that 16 g of methane reacts with 64 g of oxygen to produce 44 g of carbon dioxide and 36 g of water. Quite clearly 16+64=80 and 44+36=80 so we have proved arithmetically that we lose or gain nothing.


>> Questions <<