Balance atoms, not molecules
A balanced equation has the same number of each kind of atom on both sides. Change only the front coefficients. Changing H₂O to H₂O₂ makes a different substance. Use the smallest whole-number coefficients.
Lesson 3.3 · AQA Combined Science
Balancing chemical equations
Worked example: water
H₂ + O₂ → H₂O
1. Put 2 before H₂O to balance oxygen.
2. This makes 4 hydrogens on the right, so put 2 before H₂.
2H₂ + O₂ → 2H₂O
Check: H = 4 each side; O = 2 each side.
Build a balanced equation
Change the front numbers and compare the atom counts. Start with water, then try magnesium oxide, aluminium oxide and methane.
When two and three do not match
For Al + O₂ → Al₂O₃, use six oxygen atoms on each side: 4Al + 3O₂ → 2Al₂O₃. Then check aluminium. A helpful strategy is to balance elements in one compound first and leave H and O until later when several compounds contain them.
Combustion example
CH₄ + 2O₂ → CO₂ + 2H₂O
Carbon: 1 each side. Hydrogen: 4 each side. Oxygen: 4 each side. The coefficients give numbers of molecules or formula units, not gram ratios.
Remember
Change coefficients, never formula subscripts. Check every element and reduce to the smallest whole-number ratio.
Extra balancing pictures, worksheets and PowerPoint
Learn the method
Review the rules for balancing, diatomic molecules, coefficients and subscripts.
Count atoms on each side
Try each question before opening the answers.
Show atom-counting answers
Practise balancing
Start with one supported problem, then use the three-question and six-question sheets.
Show balancing answers
20 quick questions
Try it
Build confidence with pictures, clues and instant feedback.
Question 1 of 20Spot · Match · Explain
Check your understanding
Look at the diagram.
Case closed!
One point for each answer you get right first time.
No timer. Take your time and use a clue whenever you need one.