Electrolysis uses electricity to break down an ionic compound that is molten or dissolved. It matters in IGCSE Chemistry because it links ions, equations, reactivity and calculations in one context.
Nearly every question asks the same three things: which ions are present, which electrode each goes to, and what is formed. The half-equations then show the electron transfer.
What should I know first?
You will be more comfortable if these are secure.
- Ionic bonding and ion charges, from bonding and structure.
- Writing and balancing equations, from formulas and equations.
- Which metals are more reactive, from metals and reactivity.
- Moles and ratios, from relative masses and amounts, if you meet calculation questions.
Orienting example
Molten lead(II) bromide, PbBr₂, is electrolysed with inert electrodes.
Step 1: the ions are Pb²⁺ and Br⁻. Step 2: Pb²⁺ goes to the negative cathode and Br⁻ to the positive anode. Step 3: lead forms at the cathode, bromine at the anode.
Step 4, half-equations: Pb²⁺ + 2e⁻ → Pb and 2Br⁻ → Br₂ + 2e⁻. Adding them gives PbBr₂ → Pb + Br₂, with 1 Pb and 2 Br on each side.
Every lesson in this module is a version of those four steps.
In what order should I study the lessons?
- Identify the roles of electrodes and electrolyte: without the anode, cathode and ion movement, nothing else makes sense.
- Predict a simple molten-electrolyte product: the cleanest case, with only two ions to consider.
- Use half-equations to account for electron transfer: turns the prediction into a balanced statement about electrons.
- Compare aqueous and molten cases only within scope: adds water and the discharge choice.
- Interpret an electroplating diagram safely: applies everything to a diagram with active electrodes.
Then attempt the mixed practice set.
What are the common traps?
- Reversed electrodes. Anode is positive in electrolysis. Start from the terminal.
- Electrons in the liquid. Ions carry current in the electrolyte, and electrons move in the wires.
- Hydrogen in a molten cell. Only water supplies it.
- Forgetting Br₂, Cl₂ or O₂ are molecules. Two ions are needed per molecule.
- Electrons on the wrong side. Gain means electrons on the left, loss on the right.
How should I use the practice set?
Attempt each question with the answers covered. Mark where your reasoning first differed from the model, then use the routing table at the end to choose a lesson. The mole and equation-ratio tutor, equation balance trainer and mistake log help you check and track errors.
Electrolysis pages on this site explain the reasoning only. Real electrolysis often involves hazardous products and high temperatures, so it should only be seen or done in a supervised school setting.
Students who can follow each lesson but freeze on a new cell often need someone to watch their reasoning live. That is what our teachers offer in online one-to-one Chemistry tuition.