AQA GCSE Chemistry · Chemical Changes
Electrolysis required practical
Investigate what happens when aqueous solutions are electrolysed using inert electrodes.
Passing a current through a salt solution splits it up. A metal or hydrogen forms at the negative electrode; a halogen or oxygen forms at the positive electrode. You identify each product with a test.
Equipment
- Beaker (or electrolysis cell) and two carbon (graphite) electrodes
- Low-voltage DC power supply (about 4 V) and leads with crocodile clips
- Solutions: copper chloride, sodium chloride, copper sulfate, sodium sulfate
- Two small test tubes to collect gases
- Damp blue litmus paper and a wooden splint
Variables
- Independent you change
- The solution being electrolysed
- Dependent you measure
- The product at each electrode
- Control you keep the same
- Same inert carbon electrodes
- Same voltage
- Same concentration and volume of solution
- Same time
Method
- 1
Pour about 50 cm³ of copper chloride solution into a beaker.
- 2
Put the two carbon electrodes in, not touching, held in place with a lid or clamp.
- 3
Connect them to the DC power supply and switch on at about 4 V.
- 4
Watch each electrode. Collect any gas in an upturned test tube of the solution.
- 5
Test the products: damp blue litmus at the positive electrode; a lit splint and a glowing splint for gases.
- 6
Switch off, clean the electrodes and repeat with the other solutions.
Worked example
Products with inert carbon electrodes.
| Solution | Negative electrode (cathode) | Positive electrode (anode) |
|---|---|---|
| Copper chloride | Copper (pink-brown coating) | Chlorine (bleaches damp litmus) |
| Sodium chloride | Hydrogen (squeaky pop) | Chlorine (bleaches damp litmus) |
| Copper sulfate | Copper (pink-brown coating) | Oxygen (relights glowing splint) |
| Sodium sulfate | Hydrogen (squeaky pop) | Oxygen (relights glowing splint) |
Working
Cathode: the metal forms if it is less reactive than hydrogen (e.g. copper); otherwise hydrogen forms.
Anode: a halogen forms if the solution has halide ions (Cl⁻, Br⁻, I⁻); otherwise oxygen forms.
Cathode (copper): Cu²⁺ + 2e⁻ → Cu Anode (chlorine): 2Cl⁻ → Cl₂ + 2e⁻
Conclusion
Copper is less reactive than hydrogen, so it is deposited; sodium is more reactive, so hydrogen forms instead. Chloride solutions give chlorine at the anode; sulfate solutions give oxygen, from the water.
Risks
- Chlorine gas is toxicUse a low voltage, run it only briefly and keep the room well ventilated.
- Copper salts are harmfulWear eye protection and wash hands afterwards.
- Electrodes touching causes a short circuitKeep the electrodes apart.
Common mistakes
- Mixing up the electrodes. Positive ions (cations) go to the negative cathode; negative ions (anions) go to the positive anode.
- Saying sodium forms in sodium chloride solution. Sodium is more reactive than hydrogen, so hydrogen forms.
- Using the wrong gas test: chlorine bleaches damp litmus, oxygen relights a glowing splint, hydrogen gives a squeaky pop.
Better results
- Clean the electrodes between solutions so earlier products don’t affect results.
- Collect gases in measuring tubes to compare volumes at each electrode.
- Keep the voltage and time the same for each solution.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Chemical Changes flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.