<Dd> CH + 2 O → CO + 2 H O, </Dd> <P> where one molecule of methane (CH) and two oxygen molecules O are converted into one molecule of carbon dioxide (CO) and two of water (H O). The number of molecules as result from the reaction can be derived from the principle of conservation of mass, as initially four hydrogen atoms, 4 oxygen atoms and one carbon atom are present (as well as in the final state), then the number water molecules produced must be exactly two per molecule of carbon dioxide produced . </P> <P> Many engineering problems are solved by following the mass distribution in time of a given system, this practice is known as mass balance . </P> <P> An important idea in ancient Greek philosophy was that "Nothing comes from nothing", so that what exists now has always existed: no new matter can come into existence where there was none before . An explicit statement of this, along with the further principle that nothing can pass away into nothing, is found in Empedocles (approx . 4th century BC): "For it is impossible for anything to come to be from what is not, and it cannot be brought about or heard of that what is should be utterly destroyed ." </P>

The law of conservation of mass holds good for all of the following except