How to Balance a Chemical Equation
Updated 2026-07-12
Balancing a chemical equation means adjusting the coefficients in front of each formula so that every element has the same number of atoms on both sides. This satisfies the law of conservation of mass. You change only the coefficients, never the subscripts inside a formula.
Balancing is a counting puzzle: make the atoms of each element match on both sides. The one unbreakable rule is that you adjust the big numbers in front (coefficients), never the small numbers inside a formula (subscripts), because changing a subscript changes the substance itself.
Coefficients versus subscripts
A coefficient multiplies the whole formula, so 2 H2O means two water molecules, four H and two O. A subscript is part of the chemical identity: H2O is water, but H2O2 is hydrogen peroxide. You may only change coefficients when balancing.
Balance one element at a time
Pick an element that appears in only one place on each side, balance it with a coefficient, then move to the next. Save hydrogen and oxygen for last since they tend to appear in many compounds.
Worked example
Balance H2 + O2 giving H2O.
- Oxygen is unbalanced (2 on the left, 1 on the right). Put a 2 in front of water: H2 + O2 giving 2 H2O.
- Now hydrogen is 2 on the left but 4 on the right. Put a 2 in front of H2: 2 H2 + O2 giving 2 H2O.
- Recount: 4 H and 2 O on each side.
Answer: 2 H2 + O2 giving 2 H2O.
The common mistake
Wrong: Changing a subscript, like turning H2O into H2O2, to balance oxygen.
Fix: Never change subscripts, that changes the substance. Balance only by placing coefficients in front of the whole formula.
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Frequently asked questions
How do you balance a chemical equation?
Adjust the coefficients in front of each formula so every element has equal atoms on both sides. Balance one element at a time, and leave hydrogen and oxygen for last.
Why can't you change subscripts when balancing?
Subscripts define the substance. Changing H2O to H2O2 turns water into hydrogen peroxide. Balancing must preserve the substances, so you only change coefficients.