Before this: Chemical formulas, Conservation of mass

Balancing chemical equations

Atoms are not created or destroyed in a reaction, so each element must have the same number of atoms on both sides of the arrow.

You balance by changing coefficients (the numbers in front), never subscripts. Changing a subscript changes the substance: H₂O is water, H₂O₂ is hydrogen peroxide.

Worked example: H₂ + O₂ → H₂O

Count atoms as written:

Element	Left	Right
H	2	2
O	2	1

Oxygen is short on the right, so put a 2 in front of H₂O. That makes 4 H on the right, so put a 2 in front of H₂.

2 H₂ + O₂ → 2 H₂O

Element	Left	Right
H	4	4
O	2	2

Tips

Balance elements that appear in only one compound on each side first.
Leave elements that appear alone (like O₂ or Fe) for last, since changing them affects nothing else.
Finish by reducing to the smallest whole numbers: 4 H₂ + 2 O₂ → 4 H₂O is balanced but not in lowest terms.

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Watch Balancing by adding coefficients

Count the atoms of each element on both sides.
Oxygen does not match: 2 on the left, 1 on the right.
Never change a subscript. Add a coefficient instead.
Two water molecules. Oxygen now matches: 2 and 2.
But hydrogen does not: 2 on the left, 4 on the right.
Two hydrogen molecules. Balanced: 4 hydrogen atoms and 2 oxygen atoms on each side.

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Explore Balance by counting atoms

Change the coefficients until every element has the same count on both sides.

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