O-Level Pure Chemistry: Electrolysis — How to Predict the Products

Core idea

Electrolysis is the breakdown of an electrolyte using electricity. Cations go to the cathode (gain electrons, reduction). Anions go to the anode (lose electrons, oxidation). In exams, the real game is: which ion actually gets discharged?

Discharge rules (aqueous)

At the cathode: • More reactive metal ions (e.g. Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺) will NOT be discharged — water is reduced instead, giving H₂(g). • Less reactive metal ions (e.g. Cu²⁺, Ag⁺) will be discharged to form the metal. At the anode: • Concentrated halide ions (Cl⁻, Br⁻, I⁻) are usually discharged to form halogen. • Otherwise, hydroxide (OH⁻) from water is discharged to give O₂(g).

Key exam language

Write half‑equations with state symbols and electrons on the correct side. Example (cathode, Cu²⁺): Cu²⁺(aq) + 2e⁻ → Cu(s). Example (anode, OH⁻): 4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻. Always include physical states (aq / s / g / l) — Cambridge wants that.

Exam habits

• Circle the word ‘aqueous’ or ‘molten’ in the question first. Rules differ.
• State both product and observation (e.g. “effervescence of colourless gas that relights a glowing splint → oxygen”).
• If copper electrodes are used in CuSO₄, mention that the anode decreases in mass — that’s assessed often in O-Level Singapore papers.