Balancing a chemical equation means adjusting the coefficients (the numbers in front of molecules) so that you have the same number of each type of atom on both sides of the equation. This ensures the law of conservation of mass is followed — matter cannot be created or destroyed in a chemical reaction.
Steps
Write down the unbalanced equation with correct chemical formulas for all reactants and products. Never change the subscripts within formulas, only add coefficients in front.
Count the number of atoms of each element on both the reactant side and the product side. Make a list or table to keep track.
Start balancing with the most complex molecule or the element that appears in the fewest compounds. Add coefficients to balance one element at a time.
After adjusting a coefficient, recount all atoms because changing one coefficient affects multiple elements in that compound.
Continue adjusting coefficients and recounting until every element has the same number of atoms on both sides. Use whole numbers only.
Double-check your final equation by counting atoms of each element one more time to confirm they match on both sides.
Worked example
Let's balance: H2 + O2 → H2O. Count atoms: Left side has 2 H and 2 O; right side has 2 H and 1 O. Oxygen is unbalanced. Put a 2 in front of H2O: H2 + O2 → 2 H2O. Recount: Left has 2 H and 2 O; right has 4 H and 2 O. Now hydrogen is unbalanced. Put a 2 in front of H2: 2 H2 + O2 → 2 H2O. Final count: both sides have 4 H and 2 O. The equation is balanced.
Remember
Always balance equations by adjusting coefficients, never subscripts, and keep recounting atoms until both sides match perfectly.