A comprehensive example: the combustion of methane

Methane, with the formula CH₄, is the main component of natural gas used for cooking or heating. Its combustion is an excellent, comprehensive example for practising how to balance a chemical equation.

Setting up the reaction

Methane burns in the oxygen in the air to produce carbon dioxide and water (in the form of vapour):

CH4 + O2 -> CO2 + H2O          (not yet balanced)

Working it out, step by step

(on the left: 1 C, 4 H, 2 O)       (on the right: 1 C, 2 H, 3 O)

The carbon is already balanced (1 = 1). The hydrogen does not balance (4 versus 2): we place a coefficient of 2 in front of H₂O to double the hydrogen on the right-hand side:

CH4 + O2 -> CO2 + 2 H2O
(on the left: 1 C, 4 H, 2 O)       (on the right: 1 C, 4 H, 4 O)

The hydrogen is now balanced (4 = 4), but the oxygen is no longer balanced (2 versus 4, due to the new coefficient). We correct this by placing a coefficient of 2 in front of O₂ on the left-hand side:

CH4 + 2 O2 -> CO2 + 2 H2O
(on the left: 1 C, 4 H, 4 O)       (on the right: 1 C, 4 H, 4 O)    (balanced!)

Final check

Element	Reactants (left)	Products (right)
Carbon	1 (in CH₄)	1 (in CO₂)
Hydrogen	4 (in CH₄)	4 (in 2 H₂O)
Oxygen	4 (in 2 O₂)	4 (2 in CO₂ + 2 in 2 H₂O)

Each element is fully conserved: the equation CH4 + 2 O2 -> CO2 + 2 H2O satisfies the conservation of atoms and therefore of mass, exactly as Lavoisier required.