O-Level Chemistry Electrolysis & Redox Practical Guide

TL;DR
Electrolysis and redox investigations test your ability to wire cells safely, collect gases, and justify ionic equations, all within the Paper 3 MMO/PDO/ACE framework.
Planning marks require clear identification of electrodes, electrolyte composition, current control, and gas tests, while MMO/PDO focus on disciplined timing, bubble observation, and volumetric readings.
ACE commentary should reconcile observations with half-equations, quantify charge passed when data permits, and propose refinements that manage heat, electrode wear, and gas purity.

1 | Electrolysis in the SEAB syllabus

• Electrolysis tasks fall under qualitative inorganic analysis and gas collection within Paper 3's experimental suite, requiring candidates to deduce products at electrodes and test evolved gases (SEAB 2026 syllabus, p. 27).
• Planning/MMO/PDO/ACE strands demand correct apparatus assembly, accurate observation recording, and reflective evaluation (SEAB 2026 syllabus, pp. 25 – 26).
• Relevant apparatus includes power supplies, electrodes, gas collection tubes, measuring cylinders, bungs with delivery tubes, and balances for mass-change investigations (SEAB 2026 syllabus, p. 28).

2 | Planning the electrolysis workflow

1. Aim & cell description. Example: “Investigate the electrolysis of aqueous copper(II) sulfate using copper electrodes.”
2. Variables. Current or voltage, duration of electrolysis, electrode surface area, electrolyte concentration, and temperature control.
3. Method outline.
   • Clean electrodes with sandpaper, rinse, and measure initial masses if required.
   • Set up the power supply with correct polarity, ensuring secure connections and a stable support stand.
   • Immerse electrodes to equal depths in the electrolyte within a beaker; switch on the power and start timing.
   • Observe colour changes, gas evolution, and electrode mass changes; collect gases over water if specified.
   • Test gases using limewater (CO₂), glowing splint (O₂), lighted splint (H₂), or litmus paper (Cl₂).
4. Risk controls. Highlight hazards from concentrated solutions, potential chlorine evolution, and the need to switch off power before adjusting electrodes.

3 | MMO & PDO essentials