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Chemistry

Balance chemical equations without changing formulas

Conserve the number of atoms of each element by changing the coefficients before formulas. Changing a subscript changes the substance itself.

Question

Start with H₂ + O₂ → H₂O. The left side has two hydrogen atoms and two oxygen atoms; the right side has two hydrogen atoms and one oxygen atom. Oxygen is not balanced.

H₂ + O₂ → H₂O

Step by step

Put 2 before H₂O, then put 2 before H₂. The balanced equation is 2H₂ + O₂ → 2H₂O. Each side now has four hydrogen atoms and two oxygen atoms. The smallest whole-number coefficients are 2, 1 and 2.

2H₂ + O₂ → 2H₂O

Check your work

Do not change H₂O to H₂O₂: that changes water into hydrogen peroxide. A coefficient multiplies the entire formula. The balanced equation gives mole ratios, not the same ratio of masses; atom counts must be conserved, not molecule counts.

Try it yourself

Try N₂ + H₂ → NH₃. The balanced equation is N₂ + 3H₂ → 2NH₃: each side contains two nitrogen atoms and six hydrogen atoms.

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