Electrolysis of aqueous solutions
GCSE Chemistry (8462) · Required practical 3 — method, variables, the marks examiners report students losing.
Investigate what is produced at each electrode when aqueous solutions are electrolysed using inert electrodes.
Apparatus
- Electrolysis cell / small beaker
- Two inert graphite electrodes
- Low-voltage dc power supply
- Solutions (e.g. copper(II) sulfate, sodium chloride)
- Small test tubes, lit splint, glowing splint, damp litmus paper
Method
- 1Half-fill the cell with the solution; stand two inert graphite electrodes in it, not touching each other.
- 2Connect the electrodes to the dc power supply (one to +, one to -).
- 3Invert a small test tube over each electrode to collect any gas produced.
- 4Turn on the supply and observe what forms at each electrode.
- 5Test the gases: lit splint (hydrogen gives a squeaky pop), glowing splint (oxygen relights it), damp litmus (chlorine bleaches it white).
- 6Note any metal deposit or colour change at the cathode.
Variables
Independent
The solution being electrolysed (or its concentration)
Dependent
The products formed at each electrode
Control
- Electrode material (inert)
- Voltage
- Time
- Volume of solution
Results & processing
- Record the product at the cathode (negative) and the anode (positive) for each solution.
- Cathode: the metal forms only if it is less reactive than hydrogen (e.g. copper) — otherwise hydrogen forms. Anode: a halogen forms if a halide is present, otherwise oxygen.
Where students lose marks
Confusing which electrode is which.
Fix: Cathode is negative and attracts positive metal/hydrogen ions; anode is positive and attracts negative ions (oxygen or a halogen).
Predicting sodium metal at the cathode from sodium chloride solution.
Fix: In solution, hydrogen forms because sodium is more reactive than hydrogen; the metal is only produced if it is less reactive than hydrogen (e.g. copper).
Using the wrong gas test.
Fix: Lit splint + squeaky pop = hydrogen; glowing splint relights = oxygen; damp litmus bleached white = chlorine.
Improve the method
- Use inert (graphite) electrodes so they don't react and change the products.
- Keep the voltage and time the same when comparing different solutions.
Try it — exam-style
Concentrated sodium chloride solution is electrolysed with inert electrodes. Name the product at the cathode and at the anode, and explain the cathode product.
Describe the test and result that confirms the gas produced at the anode is oxygen.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on electrolysis of aqueous solutions?
Electrolysis marks turn on the cathode/anode rules and the gas tests — I make those mechanical, and your first lesson is free.