8 min readPathway sheet1-mark MCQ, and a 2 or 3-mark question to balance equations

Writing and balancing chemical equations

Write the reaction as a skeletal equation, then put numbers in front of the formulas until every atom is equal on both sides.

Law of conservation of massMass can neither be created nor destroyed in a chemical reaction

State symbols(s) solid, (l) liquid, (g) gas, (aq) solution in water

Golden ruleChange only the numbers in front of formulas, never the small numbers inside them

Three ways to write a reaction

Form	What it shows	Example
Word equation	Names of the reactants and products, joined by an arrow	Magnesium + Oxygen → Magnesium oxide
Skeletal equation	Formulas, but the atoms may not be equal yet	Mg + O₂ → MgO
Balanced equation with state symbols	Equal atoms on both sides, and the physical state of each substance	2Mg(s) + O₂(g) → 2MgO(s)

Balancing step by step: iron and steam

Skeletal equation: Fe + H₂O → Fe₃O₄ + H₂. Count the atoms on each side, then balance one element at a time.

1
Count the atoms. Left: Fe 1, H 2, O 1. Right: Fe 3, H 2, O 4.
Start with the formula that has the most atoms: Fe₃O₄
2
Balance oxygen: put 4 before H₂O. Now Fe + 4H₂O on the left.
Oxygen is now 4 on both sides
3
Balance hydrogen: the left now has 8 H, so put 4 before H₂ on the right.
Hydrogen is now 8 on both sides
4
Balance iron: put 3 before Fe on the left.
Iron is now 3 on both sides
5★
3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
Check: Fe 3 = 3, H 8 = 8, O 4 = 4. Water is (g) because it is steam

★ key step

Writing conditions and states

Write the condition above or below the arrow: heat, sunlight, a catalyst or a high pressure.
Example with a pressure of 340 atm: CO(g) + 2H₂(g) → CH₃OH(l)
Photosynthesis needs sunlight and chlorophyll: 6CO₂(aq) + 12H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(aq) + 6H₂O(l)
Write (aq) for a substance dissolved in water, for example NaCl(aq).
Write "+ Heat" on the product side when heat is given out.

Worked example 1: aluminium and oxygen

Problem

Balance: Al + O₂ → Al₂O₃

Working

Count. Left: Al 1, O 2. Right: Al 2, O 3.
Oxygen is 2 on the left and 3 on the right. The smallest number that both 2 and 3 go into is 6.
Put 3 before O₂ and 2 before Al₂O₃. Oxygen: 3 × 2 = 6 on the left, 2 × 3 = 6 on the right.
Aluminium on the right is now 2 × 2 = 4, so put 4 before Al.
Check: Al 4 = 4, O 6 = 6.

Answer

4Al(s) + 3O₂(g) → 2Al₂O₃(s)

Worked example 2: treat a group of atoms as one unit

Problem

Balance: AgNO₃ + CaCl₂ → AgCl + Ca(NO₃)₂

Working

Treat NO₃ as one unit, because it stays together. The right has 2 NO₃ units in Ca(NO₃)₂, so put 2 before AgNO₃.
Silver on the left is now 2, so put 2 before AgCl.
Chlorine is now 2 on each side (CaCl₂ on the left, 2AgCl on the right). Calcium is 1 on each side.
Check: Ag 2 = 2, NO₃ 2 = 2, Ca 1 = 1, Cl 2 = 2.

Answer

2AgNO₃(aq) + CaCl₂(aq) → 2AgCl(s) + Ca(NO₃)₂(aq)

In real life

Every LPG stove in an Indian kitchen obeys the law of conservation of mass. Engineers use balanced equations to work out how much air a burner needs to burn the gas fully.

Exam angle

1 marks: “State the law that makes it necessary to balance a chemical equation.” The law of conservation of mass: mass can neither be created nor destroyed in a reaction, so each element must have the same number of atoms on both sides.

2 marks: “Write a balanced equation with state symbols: sodium metal burns in chlorine gas to form solid sodium chloride.” 2Na(s) + Cl₂(g) → 2NaCl(s). Check: Na 2 = 2, Cl 2 = 2.

3 marks: “Balance: (i) Al + HCl → AlCl₃ + H₂ (ii) Fe₂O₃ + CO → Fe + CO₂ (iii) C₃H₈ + O₂ → CO₂ + H₂O” (i) 2Al + 6HCl → 2AlCl₃ + 3H₂ (ii) Fe₂O₃ + 3CO → 2Fe + 3CO₂ (iii) C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Remember it

Count, balance the biggest formula first, then count again. Change the big numbers in front, never the small numbers inside.