Lesson focus: Balanced symbol equations represent conservation of each type of atom by changing coefficientA big number in front of a formula in an equation that tells you how many of that particle there are, such as the 2 in 2H₂O., never chemical formulae.
Translate word equations into formula equations and balance them using the smallest whole-number coefficients.
Build on earlier learning
Conservation of massYear 8 established conservation of mass. This lesson expresses that conservation quantitatively in symbol equations.
You will be successful when you can…
- Distinguish coefficients and subscripts.
- Count atoms accurately.
- Balance equations systematically.
- Check smallest whole-number ratios.
Words that carry the science
- reactant
- A starting substance in a chemical reaction.
- product
- A substance formed in a chemical reaction.
- coefficient
- A number placed before a formula to show relative particle or mole amounts.
- subscript
- A small number within a formula showing atom or ion ratio.
- conservation of mass
- Total mass remains constant in a closed system because atoms are rearranged, not created or destroyed.
- state symbol
- A label such as (s), (l), (g) or (aq) that states physical form.
Balance atoms while preserving substance identity
01Formulae identify substances
A formula such as H₂O fixes the ratio of atoms in water. Changing it to H₂O₂ would name a different substance. Balance equations only by placing coefficients before formulae.
A coefficientA big number in front of a formula in an equation that tells you how many of that particle there are, such as the 2 in 2H₂O. multiplies every atom in the formula: 3H₂O contains six hydrogen atoms and three oxygen atoms.
02Balance one element at a time
Start with correct formulae, count atoms on both sides and choose coefficients. Leave elements appearing in several compounds until later when helpful. Recount after every change.
For magnesium and oxygen: Mg + O₂ → MgO becomes 2Mg + O₂ → 2MgO. Both sides contain two magnesium and two oxygen atoms.
03Equations show ratio, not a microscopic film
Coefficients give relative amounts. They do not mean the reaction occurs in one single collision containing every displayed particle, and they do not specify rate or energy change.
state symbolA letter in brackets in an equation showing the state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water. add useful conditions, but only include them when known. An equation can be atom-balanced yet still chemically implausible if the formulae or productA new substance made in a chemical reaction. are wrong.
Balancing equations
Change the big numbers in front of each formula until there are the same number of each type of atom on both sides. You cannot change the small numbers: that would make a different substance. Play tries numbers in order until it balances.
Controls
Balance aluminium oxide formation
Balance Al + O₂ → Al₂O₃.
- Balance oxygen using the lowest common multiple of 2 and 3, which is 6.
- Place 3 before O₂ and 2 before Al₂O₃.
- The products now contain 4 aluminium atoms.
- Place 4 before Al: 4Al + 3O₂ → 2Al₂O₃.
How to make the reasoning visible: Recount: Al 4 = 4 and O 6 = 6; coefficients 4:3:2 cannot be simplified.
Audit equation balance
Count atoms on each side before deciding whether an equation is balanced.
| Equation | Left totals | Right totals | Balanced? | Correction |
|---|---|---|---|---|
| H₂ + Cl₂ → 2HCl | H 2, Cl 2 | H 2, Cl 2 | Yes | None |
| Mg + O₂ → MgO | Mg 1, O 2 | Mg 1, O 1 | No | 2Mg + O₂ → 2MgO |
| 2Na + Cl₂ → 2NaCl | Na 2, Cl 2 | Na 2, Cl 2 | Yes | None |
| Al + O₂ → Al₂O₃ | Al 1, O 2 | Al 2, O 3 | No | 4Al + 3O₂ → 2Al₂O₃ |
Why is Mg + O → MgO not a valid fix?
Why simplify 2H₂ + 2Cl₂ → 4HCl?
Check your understanding
Answer from memory, then check against the suggested answer.
What can be changed when balancing?
What does a subscript show?
Why must equations balance?
Balance Fe + O₂ → Fe₂O₃.
Explain why a balanced equation does not prove the proposed reaction occurs.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Balance the equation by writing the missing numbers.
(2)N₂ + ____ H₂ → ____ NH₃
Explain each change you made.
(3)Total for this question: 5 marks
Command word: For balance and explainA command word pair: put numbers in front of formulae so both sides of the equation have the same atoms, then say why., show the atom count that motivates each coefficient.