Why equations must balance
Atoms are not created or destroyed in a chemical reaction; they are rearranged. So every atom that appears on the left of the arrow must appear the same number of times on the right. Balancing is bookkeeping for atoms.
You may only change coefficients — the big numbers in front of a formula. Changing a subscript changes the substance itself, so turning H2O into H2O2 to make the oxygen work is not balancing, it is inventing a different compound.
The order that avoids endless trial and error
Balancing feels random when you start anywhere. Follow this order and most school-level equations fall out in two or three moves.
- Count every atom on both sides first and write the tallies down.
- Balance metals next, since they usually appear in only one compound on each side.
- Balance non-metals other than hydrogen and oxygen.
- Balance hydrogen.
- Balance oxygen last, because it appears in the most compounds.
- If you end up needing a fraction, multiply every coefficient by the denominator.
Worked example: combustion of methane
Start with CH4 + O2 to CO2 + H2O. Carbon is already balanced at one each. Hydrogen is four on the left and two on the right, so put a 2 in front of water: CH4 + O2 to CO2 + 2H2O. Now oxygen is two on the left and four on the right (two in CO2, two in the two waters), so put a 2 in front of O2.
Final equation: CH4 + 2O2 to CO2 + 2H2O. Recount: carbon 1 and 1, hydrogen 4 and 4, oxygen 4 and 4. Balanced.
Worked example: a displacement reaction
Balance Al + CuSO4 to Al2(SO4)3 + Cu. Treat the sulfate group as a single unit because it survives the reaction intact. There are three sulfates on the right, so put a 3 in front of CuSO4. That gives three coppers on the right, so put a 3 in front of Cu. Aluminium is two on the right, so put a 2 in front of Al.
Final equation: 2Al + 3CuSO4 to Al2(SO4)3 + 3Cu. Grouping polyatomic ions as units is the single biggest time saver in this topic.
How to check in ten seconds
Write a two-column tally of every element, left against right, underneath your final equation. Teachers award marks for a balanced equation, not for the effort, so this check is the cheapest mark on the paper. If a tally disagrees, do not restart — adjust the coefficient for the element that is out and re-tally.