IGCSE · Practice questions · Formulae and equations

Every atom accounted for.

Six original Cambridge-style questions on formulae of ionic compounds, balancing equations, state symbols and ionic equations with spectator ions removed.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Formula first, balance second.

Ionic formula: total positive charge = total negative charge.
Brackets round a group such as OH⁻ or NH₄⁺ when there is more than one.
Balance with the numbers in front only; count O atoms last in combustion.
Ionic equation (Extended): leave out spectator ions; atoms and charges must both balance.

Recall

[4 marks]

Write the formulae of (a) calcium hydroxide, (b) ammonium sulfate, (c) aluminium oxide, (d) sodium carbonate.

(a) Ca²⁺ and OH⁻: Ca(OH)₂. ✓
(b) NH₄⁺ and SO₄²⁻: (NH₄)₂SO₄. ✓
(c) Al³⁺ and O²⁻: Al₂O₃. ✓
(d) Na⁺ and CO₃²⁻: Na₂CO₃. ✓

Calculation

[2 marks]

Balance these equations: (a) Fe + Cl₂ → FeCl₃, (b) Na + H₂O → NaOH + H₂.

(a) 2Fe + 3Cl₂ → 2FeCl₃ (2 Fe and 6 Cl on each side). ✓
(b) 2Na + 2H₂O → 2NaOH + H₂ (2 Na, 4 H and 2 O on each side). ✓

Recall

[3 marks]

Methane burns completely in oxygen to form carbon dioxide and water. Write the word equation, then the balanced symbol equation with state symbols.

methane + oxygen → carbon dioxide + water. ✓
CH₄ + 2O₂ → CO₂ + 2H₂O (1 C, 4 H and 4 O on each side). ✓
State symbols: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l). ✓

Recall

[2 marks]

Extended. Write the ionic equation, with state symbols, for the neutralisation of dilute hydrochloric acid by aqueous sodium hydroxide, and name the spectator ions.

H⁺(aq) + OH⁻(aq) → H₂O(l). ✓
The spectator ions are Na⁺ and Cl⁻. ✓

Analysis

[3 marks]

Extended. Magnesium reacts with dilute hydrochloric acid to form magnesium chloride solution and hydrogen. Write the balanced symbol equation with state symbols, then the ionic equation.

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). ✓
Cl⁻ is a spectator ion, so it is left out. ✓
Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g). Charge: +2 on each side. ✓

Explain

[2 marks]

A student balances H₂ + O₂ → H₂O by writing H₂ + O₂ → H₂O₂. Explain why this is wrong and give the correct equation.

Changing the small number changes the substance: H₂O₂ is hydrogen peroxide, not water. Only the numbers in front may be changed. ✓
2H₂ + O₂ → 2H₂O. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.