GCSE Chemistry

GCSE Chemistry electrolysis, sorted for the exam

Last updated August 2026

Electrolysis is the GCSE Chemistry topic where the rules matter more than the facts — get molten versus aqueous right, and which electrode right, and most of the marks follow. This revision game guide sorts out those rules properly, then points you at a game that covers electrolysis alongside the rest of the syllabus.

Molten or aqueous — the rule that decides everything

Electrolysis only works on ionic compounds, and only once the ions are free to move — either melted or dissolved in water. Melt a pure ionic compound like lead bromide and there are only two types of ion around: the metal (or hydrogen) ion heads to the negative electrode, the cathode, and the non-metal ion heads to the positive electrode, the anode. Simple.

Dissolve something in water instead and you've added two more ions to the mix — H⁺ and OH⁻, from the water itself — and they compete with the compound's own ions for a place at each electrode. That competition is what most exam questions are really testing.

• At the cathode — hydrogen forms unless the metal is less reactive than hydrogen, in which case the metal is deposited instead. This is why electrolysing copper sulfate solution with inert electrodes gives you copper, not hydrogen.
• At the anode — oxygen forms unless a halide ion (chloride, bromide or iodide) is present in high enough concentration, in which case you get the halogen gas instead.
• OIL RIG — oxidation is loss of electrons, reduction is gain. The anode oxidises, the cathode reduces, every time.

Where it turns up in real questions

Two applications come up again and again. Aluminium is extracted by electrolysis rather than by reduction with carbon, because aluminium is too reactive — the ore is dissolved in molten cryolite to lower its melting point and cut the huge energy cost. And electrolysis of copper sulfate solution with copper electrodes is the standard method for purifying copper: the impure anode dissolves away and pure copper builds up on the cathode, which is exactly the kind of required-practical setup worth being able to describe electrode by electrode, not just name. Fair warning if you're building your own revision timetable around this: a single pass rarely makes it stick, which is the whole case for spacing it out instead.

Questions students actually ask

Why doesn't electrolysis of copper sulfate solution produce hydrogen? Copper is less reactive than hydrogen, so copper ions are discharged at the cathode in preference to hydrogen ions — the less reactive species always wins that competition.

Why is molten sodium chloride electrolysed but not aqueous sodium chloride, when extracting sodium? In solution, water's H⁺ ions are discharged instead of sodium's, since sodium is far more reactive than hydrogen — you'd get hydrogen gas, not sodium metal. Only the molten, water-free version gives you the metal.

What actually happens at each electrode during electroplating? The object to be plated is the cathode, the plating metal is the anode, and the electrolyte is a solution of that metal's ions — the anode dissolves and the metal is deposited onto the object at the cathode.