In a chemical reaction, atoms are never created or destroyed — they are just rearranged. That single fact is the reason chemical equations have to be balanced: the same atoms that go in must come out. Balancing looks intimidating at first, but it follows a clear, repeatable method. Here is exactly how to do it.
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View feesWhy Equations Need Balancing
Law of Conservation of Mass
Coefficients vs Subscripts — the #1 Rule
You balance an equation by changing the big numbers in front of the formulas (coefficients). You must never change the little numbers inside a formula (subscripts), because that changes the substance itself.
Read the Numbers Correctly
- • 2 H₂O means 2 water molecules → 4 H atoms and 2 O atoms.
- • The 2 in front is a coefficient — you may change it.
- • The small 2 in H₂ is a subscript — you must not change it.
- • A coefficient multiplies every atom in the formula after it.
The Step-by-Step Method
The 4-Step Recipe
- 1. Write the unbalanced equation and count atoms of each element on both sides.
- 2. Balance one element at a time by adjusting coefficients (leave H and O until last).
- 3. Recount after every change — fixing one element often unbalances another.
- 4. Check all elements match, then make sure the coefficients are the smallest whole numbers.
Balance: H₂ + O₂ → H₂O (formation of water)
Count atoms on each side
Balance oxygen with a coefficient
Recount — hydrogen is now off
Balance hydrogen
Final check
Balance: CH₄ + O₂ → CO₂ + H₂O (burning methane)
Count atoms
Balance hydrogen first
Recount oxygen (do it last)
Balance oxygen
Final check
Handy Tips for Trickier Equations
Three Shortcuts
- • Treat polyatomic ions as a unit. If a group like SO₄ appears unchanged on both sides, balance it as one block instead of element by element.
- • If you get a fraction, double everything. A coefficient of 1½ becomes a whole number when you multiply every coefficient by 2.
- • Balance lone elements last. A pure element like O₂ or Na is easy to adjust at the end without disturbing anything else.
Balance: Na + Cl₂ → NaCl
Count atoms
Balance chlorine
Recount sodium and fix it last
Final check
Quick Reference
| Do | Don’t |
|---|---|
| Change coefficients (numbers in front) | Change subscripts (numbers inside formulas) |
| Balance one element at a time | Try to balance everything at once |
| Recount after each change | Assume earlier elements stay balanced |
| Reduce to smallest whole numbers | Leave fractions in the final answer |
- Atoms are conserved — the same atoms in must come out (law of conservation of mass).
- Balance by changing coefficients, never subscripts.
- A coefficient multiplies every atom in the formula after it.
- Balance one element at a time, leaving H and O until last.
- Recount after every change.
- Reduce the final coefficients to the smallest whole numbers.
Practice Problems
- 1
Balance: N₂ + H₂ → NH₃
Hint: Balance nitrogen first, then hydrogen — you may need a 2 and a 3.
- 2
Balance: Mg + O₂ → MgO
Hint: Oxygen is the problem. A 2 in front of MgO and a 2 in front of Mg fixes it.
- 3
Balance: C₃H₈ + O₂ → CO₂ + H₂O (burning propane)
Hint: Balance C, then H, then O last.
- 4
Balance: Fe + O₂ → Fe₂O₃
Hint: Try doubling everything if you get a fraction for oxygen.
- 5
Why is changing H₂O to H₂O₂ never allowed when balancing?
Hint: Think about what the substance becomes.
Next: Reaction Types and Mole Ratios
Once you can balance equations, the coefficients unlock the next big idea: mole ratios, which let you calculate exactly how much product a reaction makes. If balancing still feels like guesswork, start with a consultation and we will turn it into a method you can trust.
