Balancing a chemical equation

A chemical equation uses symbols to represent what happens during a reaction. To comply with Lavoisier’s law of conservation of mass, it must be balanced.

The golden rule

An equation is balanced when it contains the same number of atoms of each element on either side of the arrow. This number is adjusted solely by means of coefficients placed in front of the formulae — never by changing the subscripts within a formula, as this would alter the very nature of the substance.

A step-by-step example

Let’s start with the unbalanced reaction between dihydrogen and dioxygen to form water:

H2 + O2 -> H2O          (not yet balanced)

Let’s count the atoms:

(on the left: 2 H, 2 O)       (on the right: 2 H, 1 O)

The oxygen doesn’t balance: 2 on the left, only 1 on the right. We add a coefficient of 2 in front of H₂O to double the oxygen on the right:

H2 + O2 -> 2 H2O
(on the left: 2 H, 2 O)       (on the right: 4 H, 2 O)

Now it is the hydrogen that no longer balances (2 versus 4). We add a coefficient of 2 in front of H₂ on the left:

2 H2 + O2 -> 2 H2O
(on the left: 4 H, 2 O)       (on the right: 4 H, 2 O)    (balanced!)

The method to remember

1. Count the atoms of each element on each side.
2. Identify the element that doesn’t match.
3. Add a coefficient to correct it, without changing the subscripts in the formulae.
4. Double-check all the elements, as correcting one may throw another out of balance.
5. Repeat until there are equal numbers of each element on both sides.