What Is Electrolysis in O Level Chemistry?
Electrolysis is the process of using electrical energy to drive a non-spontaneous chemical reaction. You pass electricity through a liquid called an electrolyte, causing chemical changes at two electrodes — the cathode and the anode. The electrolysis O level syllabus covers two main scenarios: electrolysis of molten compounds and electrolysis of aqueous solutions. Understanding which ions are present and which are preferentially discharged is the core skill examiners test.5 Facts About Electrolysis Every O Level Student Must Know
Fact 1: You Need an Electrolyte, Two Electrodes, and a Power Supply
For electrolysis to occur, three things must be in place:- Electrolyte — a substance that conducts electricity when molten or dissolved in water, because it contains mobile ions. Examples: sodium chloride solution, copper sulfate solution, molten lead bromide
- Cathode — the negative electrode, connected to the negative terminal of the power supply
- Anode — the positive electrode, connected to the positive terminal of the power supply
Fact 2: Cations Move to the Cathode, Anions Move to the Anode
Ion movement is one of the most fundamental rules in electrolysis O level chemistry — and one of the most frequently tested:- Cations (positive ions) are attracted to the cathode (negative electrode) — they gain electrons and are reduced
- Anions (negative ions) are attracted to the anode (positive electrode) — they lose electrons and are oxidised
Fact 3: Electrolysis of Molten Compounds Is Straightforward
When a pure ionic compound is melted (e.g. molten lead bromide, PbBr₂), only the ions from that compound are present. Predicting what forms at each electrode is then straightforward:| Electrode | Ion Discharged | Product | Reaction Type |
|---|---|---|---|
| Cathode (−) | Pb²⁺ (lead ions) | Lead metal (grey deposit) | Reduction |
| Anode (+) | Br⁻ (bromide ions) | Bromine gas (brown fumes) | Oxidation |
- Cathode: Pb²⁺ + 2e⁻ → Pb
- Anode: 2Br⁻ → Br₂ + 2e⁻
Fact 4: Aqueous Solutions Involve Selective Discharge
When an ionic compound is dissolved in water, H⁺ and OH⁻ ions from water are also present. At each electrode, competing ions exist, and only one will be preferentially discharged. The rules for selective discharge are: At the cathode (reduction):- If the metal is below hydrogen in the reactivity series → the metal ion is discharged (e.g. Cu²⁺ → Cu)
- If the metal is above hydrogen in the reactivity series → hydrogen is discharged instead (H⁺ → H₂)
- If halide ions (Cl⁻, Br⁻, I⁻) are present in high concentration → the halide is discharged
- Otherwise → oxygen is discharged from OH⁻ ions
Fact 5: Electroplating Is an Application of Electrolysis
O Level students must understand how electrolysis is applied in electroplating — coating one metal with a thin layer of another to improve appearance or prevent corrosion. In electroplating:- The object to be plated is used as the cathode
- The plating metal is used as the anode
- The electrolyte is a solution of a salt of the plating metal
How to Study Electrolysis Effectively for O Level Chemistry
Master the Half-Equations
Every electrolysis question will either ask you to write a half-equation or identify products. Practise writing balanced half-equations for common ions: Cu²⁺, Pb²⁺, H⁺, Cl⁻, Br⁻, OH⁻, SO₄²⁻. If these are automatic, you can handle any exam variation.Always Identify All Ions Present First
Before answering any electrolysis question, write down every ion present in the electrolyte — including H⁺ and OH⁻ from water for aqueous solutions. This habit prevents the most common mistake: forgetting that water contributes ions in aqueous electrolysis.Link Electrolysis to Redox and the Reactivity Series
Electrolysis connects directly to redox chemistry (OIL RIG) and the reactivity series (for predicting cathode products in aqueous solutions). For tips on how to connect topics across the whole syllabus, read our guide on top tips to score well in O Level Chemistry. If you are also working through acids, bases and salts and the mole concept, both appear alongside electrolysis in the higher-order questions of Paper 2.What Most Students Get Wrong in Electrolysis Exam Answers
The most common mistakes are: confusing cathode and anode, forgetting H⁺ and OH⁻ ions in aqueous solutions, and writing unbalanced half-equations. These errors are entirely avoidable with the right preparation. At IONX Labs, O Level Chemistry classes work through electrolysis systematically — from the basic setup to selective discharge rules and electroplating applications — using real past paper questions to build the exact skills examiners are looking for.Get Help With Electrolysis O Level Chemistry
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Frequently Asked Questions
The cathode is the negative electrode, connected to the negative terminal of the power supply. Cations (positive ions) move towards it, gain electrons, and are reduced. The anode is the positive electrode, connected to the positive terminal. Anions (negative ions) move towards it, lose electrons, and are oxidised. A useful memory aid: the CAThode attracts CAT-ions.
In solid ionic compounds, ions are held in a fixed lattice structure and cannot move. Because ions cannot move, they cannot carry charge to the electrodes — so no electrical conduction occurs and no electrolysis takes place. Electrolysis only works when the ionic compound is either molten (ions are free to move in the liquid) or dissolved in water (ions are separated and free to move through the solution).
Water partially ionises to produce H⁺ ions and OH⁻ ions. In any aqueous electrolysis, these ions are always present alongside the ions from the dissolved compound. At the cathode, H⁺ competes with the metal cation — if the metal is above hydrogen in the reactivity series, H₂ gas is discharged instead of the metal. At the anode, OH⁻ competes with anions — if no high-concentration halide is present, oxygen is discharged from OH⁻.
For cathode (reduction) half-equations, add electrons to the left side: e.g. Cu²⁺ + 2e⁻ → Cu. For anode (oxidation) half-equations, add electrons to the right side: e.g. 2Cl⁻ → Cl₂ + 2e⁻. Check that charge and atoms balance on both sides. The number of electrons must match the charge on the ion — Cu²⁺ requires 2 electrons, Al³⁺ requires 3 electrons. Unbalanced half-equations are one of the most common mark losses in this topic.
In electroplating, the object to be coated is used as the cathode (negative electrode), the plating metal is the anode (positive electrode), and the electrolyte is a solution containing ions of the plating metal. Metal ions from the anode dissolve into the solution, while metal ions from the solution are deposited onto the cathode (the object being plated). The anode gradually dissolves and the mass of the cathode increases by the same amount.