Atoms in, atoms out
In any chemical reaction, atoms are rearranged but never created or destroyed — that is the law of conservation of mass. Balancing an equation is putting numbers (coefficients) in front of formulas so the count of every element on the left matches the count on the right.
Anatomy of an equation
2 H₂ + O₂ → 2 H₂O. The numbers in front are coefficients — multiply the whole formula. Subscripts (the small numbers inside a formula) cannot change — that would change the substance.
The balancing routine
1. Count atoms of each element on both sides. 2. Balance the most complex molecule first. 3. Save free elements (H₂, O₂, Na...) for last. 4. Use only whole-number coefficients in the final answer.
Why it matters
A balanced equation is the recipe behind stoichiometry. Without balance, you cannot calculate yields, limiting reagents or anything else quantitative.
Common gotchas
Polyatomic ions (SO₄²⁻, NO₃⁻) often pass through unchanged — balance them as a unit. If a coefficient comes out fractional (½), multiply everything by the denominator.
Worked example: combustion of propane
C₃H₈ + O₂ → CO₂ + H₂O
- Carbon: 3 on left, 1 on right → put 3 in front of CO₂. C₃H₈ + O₂ → 3 CO₂ + H₂O
- Hydrogen: 8 on left, 2 on right → put 4 in front of H₂O. C₃H₈ + O₂ → 3 CO₂ + 4 H₂O
- Oxygen: now 6 + 4 = 10 on right, 2 on left → put 5 in front of O₂. C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
- Recount: C 3=3 ✓, H 8=8 ✓, O 10=10 ✓. Balanced.
Hands-on tools
Adjust coefficients and watch the atom counts. Or paste any equation into the auto-balancer.
Pick coefficients, watch the atoms
Tap an equation, then use the +/− buttons to set each coefficient. The atom counter shows when both sides match.
Speed drill
Type the four coefficients separated by commas. Hit Enter to check.
Automatic balancer
Type any equation. Use → or = between sides; + between species. Subscripts written as digits, like H2O for water. Parentheses ok: Ca(OH)2.
Quiz
Flashcards
Tap a card to flip. ← / → keys to navigate.
Daily challenge
Five equations to balance. Same set worldwide, refreshes at midnight UTC. One shot per day.
For teachers
Print-ready worksheet, answer key, teaching tips and standards alignment.
Teaching tips
Standards alignment
Reference
Common equations to know
Glossary
Photo gallery
Images sourced from Wikipedia.