The Conservation of Mass in a Chemical Reaction:

Chemical reactions may seem to only take place in books and labs, but they are in fact taking place all around us.  Every time
you metabolize a meal, light a match, start a car, or even think a thought, chemical reactions are responsible.  Reactions, which
are often initiated by catalysts, take place when new molecular bonds are created or broken to link or break apart different
elemental atoms.  One such reaction is the reaction between hydrogen and oxygen to create water.  In chemistry you represent
a reaction by writing the reactants (the substances before the reaction) to the left of an arrow, and the products (the product of
the reaction) to the right of the arrow.  If you know that water is H2O, then this might lead you to represent the reaction like

H + O   H2O

But this is wrong.  Because atoms like to have full valence shells, H and O atoms by themselves are very rare.  In fact, both
hydrogen and oxygen react with themselves to create the molecules H2 and O2, respectively.  It is much more common to see
O and H atoms paired up in molecules of two than it is to see them alone.  With this knowledge, one might assume then that the
reaction between hydrogen and oxygen would look like this:

H2 + O2  H2O

This actually closer but still not quite right.  This equation tells us that one hydrogen molecule (composed of two H atoms) and
one oxygen molecule (composed of two O atoms) will form a single water molecule with 2 H atoms and 1 O atom.  In this
equation we seem to have lost one of the O atoms.  But the principle of the conservation of mass tells us that no atoms are lost
in chemical reactions.  In other words the number of atoms on the left side of a chemical equations must be balanced with the
number of atoms on the right side.  In reality the O atom that we lost, under most circumstances, would then bond with a
second molecule of hydrogen in essence creating two different water molecules.  So the reaction is written:

2H2 + O2  2H2O

In this reaction equation, the number in front of the molecule (which is called a coefficient) designates the number of molecules
that participate in the reaction.  To conclude, we can read this as saying: two separate hydrogen molecules (with 2 H atoms
each) combine with a single oxygen molecule (with 2 O atoms) to create 2 individual molecules of water (each with two H
atoms and a single O atom).
Organization for the Advancement of   
Interdisciplinary Learning