Discharge!

Discharge! Electrolysis of aqueous solutions, one ion at a time

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

What happens when an aqueous solution is electrolysed?

When an ionic compound dissolves in water, its ions are free to move, so the solution is an electrolyte. Pass a current through it and the ions move to the electrodes: positive ions move to the negative electrode (the cathode) and negative ions move to the positive electrode (the anode), where they are discharged. For AQA GCSE Chemistry (spec point 4.4.3.4) the twist is the water. Water molecules break down a little, producing hydrogen ions (H⁺) and hydroxide ions (OH⁻), so two kinds of positive ion arrive at the cathode and two kinds of negative ion arrive at the anode. Only one of each is discharged.

Discharge! makes that competition visible. You run Cell 3 on the night shift at Volta Works, a 3D electrolysis cell with two inert carbon electrodes and a gas collection tube over each one. You watch every ion drift through the solution, see each H⁺ and OH⁻ come from a water molecule splitting in two, and decide, ion by ion, which one is discharged at each electrode.

The two rules: which ion is discharged?

At the cathode, hydrogen is produced if the metal is more reactive than hydrogen. Sodium, potassium and magnesium are all more reactive than hydrogen, so their solutions give bubbles of hydrogen gas and the metal ions stay in the solution. Copper and silver are less reactive than hydrogen, so the metal is produced instead: you watch pink-brown copper or grey silver crystals build up on the cathode, and the blue colour of copper sulfate fades as the Cu²⁺ ions are used up.

At the anode, oxygen is produced unless the solution contains halide ions, when the halogen is produced. Chloride ions give chlorine gas, bromide ions give bromine (which dissolves and turns the solution orange-brown) and iodide ions give iodine (a brown solution). Sulfate and nitrate ions are not discharged, so their solutions give oxygen, made from the hydroxide ions. Brine (sodium chloride solution) gives hydrogen at the cathode and chlorine at the anode, and leaves sodium hydroxide in the solution. Sodium nitrate solution gives hydrogen and oxygen, both from water, with about twice as much hydrogen as oxygen in the tubes.

Required practical 3, gas tests and half equations

The game follows required practical 3: investigating what happens when aqueous solutions are electrolysed using inert electrodes, including developing a hypothesis. Before each new solution you predict the product at each electrode, using the hypothesis that the products depend on how reactive the metal is and whether there are halide ions, and the batch report tells you if the results support your prediction. The solutions include the ones used in the practical: copper sulfate, sodium chloride, copper chloride, sodium bromide and sodium nitrate.

When a gas tube fills, you lift it out and test the gas. Hydrogen: hold a burning splint at the open end and it burns with a pop. Oxygen: a glowing splint relights. Chlorine: damp litmus paper is bleached and turns white (chlorine is toxic, so this is done in a fume cupboard or a well-ventilated room). Every discharge also shows its half equation, and each batch report has a Higher tier question: at the cathode positive ions gain electrons (reduction), for example 2H⁺ + 2e⁻ → H₂ and Cu²⁺ + 2e⁻ → Cu; at the anode negative ions lose electrons (oxidation), for example 2Cl⁻ → Cl₂ + 2e⁻ and 4OH⁻ → O₂ + 2H₂O + 4e⁻. Foundation tier students can skip these.

How to play

Each ion waits in a glowing ring at its electrode for a moment. While the winning ion is in the ring, press DISCHARGE: A or the left arrow for the cathode, L or the right arrow for the anode, the big buttons, or tap that side of the tank. Let the other ion go: it stays in the solution. Discharge the wrong ion and the cell faults, costing points and opening a short explanation with a link to the lesson. The first batch is a slow training run with Sam, the shift engineer, showing you the first ions. Then the ions come faster through brine, copper chloride, sodium bromide and sodium nitrate, and the shift ends with an 80-second rush hour where the solution changes every few ions. A game takes about 6 to 9 minutes, works with a mouse, keyboard or touch screen, and your best score is saved so you can play again.