Science required practicals
How do you do the electrolysis required practical?
Pour a salt solution such as copper(II) chloride into a beaker, dip in two carbon electrodes that do not touch, and connect them to a low-voltage direct current (DC) supply. Watch each electrode and test any gas. At the negative electrode you get the metal if it is less reactive than hydrogen, otherwise hydrogen. At the positive electrode you get a halogen if the solution has halide ions, otherwise oxygen.
Last updated 23 September 2026 · Written and fact-checked by the GCSEwiz team
What does the electrolysis practical investigate?
What forms at each electrode when a solution of an ionic compound is electrolysed using inert electrodes, usually carbon (graphite) rods. Inert means the electrodes carry the current without reacting themselves.
It is a required practical on both GCSE Chemistry and Combined Science: Trilogy. AQA wants it run as an investigation with a hypothesis, so before you switch on, predict what will form at each electrode and say why.
The products vary because water brings ions of its own. A little of the water breaks down into hydrogen ions (H⁺) and hydroxide ions (OH⁻), so at each electrode two kinds of ion are competing, and the rules further down say which one turns into an element.
What equipment do you need?
You need a beaker of salt solution with two carbon rods held apart in it, wired to a low-voltage DC power supply. For testing the products you need damp blue litmus paper and a couple of wooden splints.
- Copper(II) chloride solution and sodium chloride solution (copper(II) sulfate makes a useful third)
- A 100 cm³ beaker
- Two carbon rod electrodes
- A Petri dish lid with two holes, to hold the rods apart
- Two leads with crocodile clips
- A low-voltage DC power supply
- Blue litmus paper and forceps
- Small test tubes and wooden splints for testing gases
- Eye protection
What is the method?
Electrolyse one solution for a few minutes, record what happens at each electrode and test any gas, then repeat with a different salt solution and compare.
- Pour about 50 cm³ of copper(II) chloride solution into the beaker.
- Put the Petri dish lid on the beaker and push a carbon rod through each hole. The rods must not touch.
- Connect the rods to the DC terminals of the power supply, set it to 4 V and switch on.
- Watch both electrodes. Record any bubbles of gas, and any solid that coats a rod.
- Use forceps to hold a piece of damp blue litmus paper next to the positive electrode. If it is bleached white, the gas is chlorine.
- To test another gas, collect it in a small test tube filled with the solution and held upside down over the electrode. A lighted splint gives a squeaky pop with hydrogen; a glowing splint relights in oxygen.
- Switch off after about 5 minutes. Rinse the apparatus, use clean rods and repeat with sodium chloride solution.
What are the variables?
You change the salt in the solution and observe what forms at each electrode. The circuit and the amount of solution stay the same, so any difference in the products comes from the salt.
| Variable | In this practical |
|---|---|
| Independent variable | The salt in the solution, for example copper(II) chloride or sodium chloride |
| Dependent variable | What forms at each electrode, judged by what you see and by the gas tests |
| Control variable | The voltage (for example 4 V) and how long the current flows |
| Control variable | The electrodes: the same inert carbon rods, the same distance apart |
| Control variable | The volume and concentration of each solution |
How do you work out and present the results?
The results are observations, so there is no calculation. Record them in a table with a row for each solution and a column for each electrode, then use two rules to explain them.
At the negative electrode (the cathode), a metal less reactive than hydrogen, such as copper, coats the electrode. A metal more reactive than hydrogen, such as sodium, stays in solution and hydrogen gas forms instead. At the positive electrode (the anode), halide ions (chloride, bromide or iodide) give the halogen. Without a halide, oxygen forms.
On the Higher tier you also write half equations. Copper: Cu²⁺ + 2e⁻ → Cu. Hydrogen: 2H⁺ + 2e⁻ → H₂. Chlorine: 2Cl⁻ → Cl₂ + 2e⁻. Oxygen: 4OH⁻ → O₂ + 2H₂O + 4e⁻. The table below shows what four common solutions give.
| Solution | Negative electrode (cathode) | Positive electrode (anode) |
|---|---|---|
| Copper(II) chloride | Pink-brown coating: copper | Bubbles that bleach damp blue litmus: chlorine |
| Sodium chloride | Bubbles that give a squeaky pop: hydrogen | Bubbles that bleach damp blue litmus: chlorine |
| Copper(II) sulfate | Pink-brown coating: copper | Bubbles that relight a glowing splint: oxygen |
| Sodium sulfate | Bubbles that give a squeaky pop: hydrogen | Bubbles that relight a glowing splint: oxygen |
How do you make it accurate and safe?
Chlorine is toxic, and two of the solutions here make it. Keep the voltage low (4 V is plenty), switch off after about 5 minutes, and work in a well-ventilated room without leaning over the beaker. Copper compounds are harmful if swallowed, so wear eye protection and wash your hands afterwards.
Keep the rods apart. If they touch, the current takes a short cut straight between them and no electrolysis happens.
For clear results, use clean rods for each solution, so copper left from one run does not confuse the next. Test each gas as soon as you have collected some, and look closely at each rod: a thin coating of copper is easy to miss at first.
What do exam questions ask about it?
A typical question gives you a solution you have not used and asks what forms at each electrode, and why. The answer always comes down to two comparisons: the metal against hydrogen, and whether a halide is there. You may also be asked to name a gas from a test result, label a diagram of the cell, or complete a half equation on the Higher tier. For example:
"Potassium iodide solution is electrolysed using inert electrodes. Name the product at each electrode and explain why potassium is not produced. (3 marks)"
Work it through with the rules: potassium is more reactive than hydrogen, so hydrogen forms at the negative electrode; iodide is a halide, so iodine forms at the positive electrode.
Common mistakes
Two of these come from skipping one of the rules, so say both to yourself before you answer.
- Predicting sodium or potassium at the negative electrode. Both are more reactive than hydrogen, so hydrogen forms instead.
- Mixing up the electrodes. Positive ions move to the negative electrode (cathode); negative ions move to the positive electrode (anode).
- Testing for chlorine with dry litmus paper. It has to be damp to be bleached.
- Forgetting oxygen. If the solution has no halide ions, the positive electrode gives oxygen.
- Putting the electrons on the side where they do not belong. Positive ions gain electrons, so e⁻ goes on the left; negative ions lose them, so e⁻ goes on the right.