A chemical equation is scientific shorthand which describes what is happening during a chemical reaction. After a chemical equation is correctly balanced, it shows exactly how much of each reactant is required to produce a fixed amount of each product. This ratio will always remain the same, no matter how large or small the amounts used.
The secret to balancing a chemical equation is that atoms cannot disappear or appear out of nowhere. Every balanced chemical equation will have exactly the same number of each type of atom on one side as on the other side. This pattern is called the law of conservation of mass.
(Nuclear equations are different because they transmute one kind of element into another. In chemistry, the elements always remain unchanged. Only their combinations, charges, and bonds differ.)
The trick to balancing chemistry equations is to always write down the unbalanced equation first. No matter how familiar the equation is, things can get interesting when trying to mentally juggle ratios. Let's try a simple one:
H2O -> H2 + O2
In other words, when an electric current is run through water (H2O), the atoms will split apart, then recombine to create hydrogen gas (H2) and oxygen gas (O2). (At temperatures above 2400 degrees C, the electrical current is not needed.) The reactants are always listed on the left side of the equation, and the products are always on the right side of the equation. The "2" which goes after the atom means that two of those atoms are involved in a molecule. It would normally be written in subscript. If only one atom of its type is involved in a molecule, the number is not needed.
But something does not look right. There are two hydrogen atoms on the left and two hydrogen atoms on the right; but the oxygen atoms don't match up. There are two oxygen atoms on the right, but only one on the left. Since atoms cannot just disappear, this equation is not yet balanced.
An extra atom of oxygen is needed, so let's start by doubling the water molecule:
2(H2O) -> H2 + O2
(You don't need the parenthesis. I just put it in to keep things clear, since I am writing without access to subscripts. The number which doubles the water molecule always goes in front, and is never written in subscripts.)
Now there are two oxygen atoms on the left and two oxygen atoms on the right. So far, so good. Unfortunately, now we have a different problem. There are *four* hydrogen atoms on the left side now, and only two on the right. We need two more hydrogen atoms on the right: and we can get them by doubling the hydrogen gas on the right.
2(H2O) -> 2(H2) + O2
The numbers of each type of atom now match on both sides. The chemical equation is balanced. Now we can tell that two water molecules produce two hydrogen molecules and one oxygen molecule.
We could have doubled everything again and said that four water molecules produce four hydrogen molecules and two oxygen molecules: but we always want to end up with the least number of atoms that will balance the equation.
For an equation which involves more types of atoms, exactly the same process applies, but it takes a little longer. Start by balancing one type of atom, then balance the second and the third until the equation is completely balanced. For example, when burning methane, the chemical equation is:
CH4 + O2 -> CO2 + H2O
In other words, when methane (CH4) burns in (combines with) oxygen (O2), it produces carbon dioxide (CO2) and water (H2O).
Carbon is already balanced: one atom per side. Otherwise that would have been the first element to tackle. The right side needs more hydrogen, so let's double the water molecule:
CH4 + O2 -> CO2 + 2(H2O)
Now the left side needs more oxygen, which can be accomplished by doubling the oxygen molecule:
CH4 + 2(O2) -> CO2 + 2(H2O)
The chemical equation is now balanced, and shows that each molecule of methane combines with two oxygen molecules to produce one molecule of carbon dioxide and two molecules of water. No matter how large the amounts used, the ratios will always remain the same. An efficient engine which burns methane will produce twice as much water in its exhaust.