Balancing Chemical Equations
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Balancing a chemical equation means adjusting only coefficients, never subscripts, until every element's atom count matches on both sides.
Propane's combustion, C3H8 + O2 → CO2 + H2O, is balanced by first matching carbon with a coefficient of 3 on CO2, then hydrogen with a coefficient of 4 on H2O, which fixes ten oxygen atoms on the product side. Placing a coefficient of 5 on O2 supplies exactly ten oxygen atoms on the reactant side, giving the balanced equation C3H8 + 5O2 → 3CO2 + 4H2O, checked by counting three carbon, eight hydrogen, and ten oxygen atoms on each side.
Changing a subscript to balance an element, such as writing O2 as O4, silently changes which substance the formula names and destroys the equation's meaning.