A chemical equation describes what happens in a chemical reaction. It has reactants on the left side, an arrow indicating reaction direction, and products on the right side.
Per the Law of Conservation of Mass, matter cannot be created or destroyed. Consequently, a chemical equation must be balanced: the number of atoms of each element on the reactant side must equal the number of atoms of those same elements on the product side.
The Golden Rule of Balancing
"Change only the coefficients (the numbers in front of the formulas). Never change the subscripts (the numbers within the formulas)!"
If you change the subscripts, you change the identity of the chemical itself. For example, changing H₂O to H₂O₂ balances Oxygen atoms, but it converts water into hydrogen peroxide!
Example Walkthrough: Combustion of Propane
Let's balance the combustion reaction:C₃H₈ + O₂ ──> CO₂ + H₂O
1. Count reactant atoms: C = 3, H = 8, O = 2.
2. Count product atoms: C = 1, H = 2, O = 3 (2 from CO₂ + 1 from H₂O).
3. Balance Carbon: Put coefficient 3 in front of CO₂:C₃H₈ + O₂ ──> 3CO₂ + H₂O
4. Balance Hydrogen: Put coefficient 4 in front of H₂O (giving 4 × 2 = 8 H):C₃H₈ + O₂ ──> 3CO₂ + 4H₂O
5. Balance Oxygen: The right side now has 3 × 2 = 6 (from CO₂) + 4 (from H₂O) = 10 Oxygen atoms. Put coefficient 5 in front of reactant O₂:C₃H₈ + 5O₂ ──> 3CO₂ + 4H₂O
The equation is now balanced!