Balancing equations

When wood burns in a fireplace, it looks like it simply disappears. But nature's scales do not lie: not a single atom is lost, afterwards they are all inside smoke, gas and ash. If you could catch all of that and weigh it, you would even get more than the wood weighed before, because oxygen from the air reacts along with it. This principle is the key to writing reaction equations correctly.

Atoms never vanish

In a chemical reaction, atoms are only rearranged, none is created and none disappears. This is called conservation of mass. For a reaction equation this means: the same number of each type of atom must stand on the left and right of the arrow. If there are 4 hydrogens on the left, there must be 4 on the right too. An equation where this holds is called balanced.

Change coefficients, not formulas

To balance, you may only adjust the coefficients, the big numbers in front of the formulas. The formulas themselves are off limits: you must never turn H₂O into H₃O, because that would be a completely different substance. A coefficient only says how many molecules take part, the formula says which substance it is. And only the amount may be changed. For counting, the rule is: the coefficient applies to the whole formula, so you multiply it by every subscript inside. That is why 2 H₂O holds 2 times 2 = 4 hydrogen atoms and 2 times 1 = 2 oxygen atoms.

Step by step to balance

Take the reaction of hydrogen with oxygen: H₂ + O₂ → H₂O. Count: the left has 2 H and 2 O, the right has 2 H and only 1 O. The oxygen does not match. Put a 2 in front of the water on the right: H₂ + O₂ → 2 H₂O. Now the oxygen fits, but the right has 4 H and the left only 2. So put a 2 in front of the hydrogen on the left: 2 H₂ + O₂ → 2 H₂O. Recount: 4 H and 2 O on both sides. Done, the equation is balanced.