Question 1 (2 marks)
Knowledge
What are the charges of the cathode and anode in electrolysis?
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Cathode negative (1); anode positive (1).
Required Practical 09: Electrolysis | AQA GCSE Science | The Science Department
About this practical and its aim
Aim
Investigate electrolysis of aqueous solutions using inert electrodes and predict products.
Introduction
By completing this practical, you will be able to observe the products. When copper (II) sulphate solution is electrolysed, copper is produced at the cathode (because copper is less reactive than hydrogen) and oxygen is produced at the anode (because hydroxide ions are less stable than sulphate ions). You will be able to see a new layer of copper forming on the cathode. A teacher may demonstrate the same with copper (II) chloride. Here chlorine will form at the anode as chlorine is less reactive than the sulphate. This is rarely done as a class practical due to the intensity of the chlorine smell.
The science behind the practical
Positive ions move towards the cathode where reduction occurs; negative ions move towards the anode where oxidation occurs. Products depend on ions present and electrode material.
Key things to remember
Use inert electrodes; connect to a low-voltage d.c. supply; make and test predictions; distinguish cathode and anode products.
Equipment and apparatus
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Electrolysis cell
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inert graphite electrodes
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low-voltage d.c. power supply
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leads and clips
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suitable aqueous electrolytes
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gas test equipment where authorised
Method: How to carry out the experiment
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Attach two clean strips of copper, one to each of the crocodile clips.
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Lower the frame into the copper sulphate solution ensuring that the clips do not touch the solution and that the copper strips are not touching.
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Attach cables from the crocodile clips to the power supply's DC connections.
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Ensure that the voltage does not exceed 6V then turn of the power supply and begin making observations.
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You may also measure the dry mass of each electrode before and after a set time to calculate the production rate of copper.
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Your teacher may allow you to try electroplating. To do this, swap the strip of copper on the cathode for a clean coin and run the experiment further. (Note, this may stop your coin from being recognised in a vending machine so use low value coins).
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All copper (II) sulphate solution must be poured into the bucket at the end, it must not go down the sink.
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Variables and fair testing
| Type of variable | What is measured or controlled? |
|---|---|
| Independent variable | Electrolyte or dissolved ions investigated |
| Dependent variable | Products and observations at each electrode |
| Control variables | Concentration; voltage; electrode material; temperature; time |
Safety and managing risks
Follow your teacher's instructions and your school's risk assessment. This page is not a substitute for a formal risk assessment.
| Hazard | Risk | Precaution |
|---|---|---|
Electrical apparatus and aqueous electrolyte |
Spills or short circuits |
Use low-voltage supply; keep leads dry and check setup before switching on. |
Possible gas products |
Irritant/toxic gases for some electrolytes |
Use only teacher-approved electrolytes and ventilation; test gases only under assessed conditions. |
Results, calculations and analysis
Recording, presenting and interpreting results
Record products, gas tests and electrode observations for each electrolyte; include half-equations where appropriate.
Calculations
Possible extension: calculate simple amounts from measured volumes or mass changes where supplied.
Conclusions, accuracy and evaluation
Drawing a conclusion
The products depend on which ions are discharged; avoid assuming copper electrodes are inert.
Evaluating the investigation
Use a control electrolyte and compare expected with observed products; explain possible competing reactions.
More to learn
Inert versus copper electrodes
The required practical uses inert electrodes. Electrolysis with copper electrodes can give different products and belongs in a clearly labelled extension.
Common exam mistake
Positive ions go to the negative cathode; negative ions go to the positive anode.
GCSE examination practice
Answer each question in full before opening the model answer. Check your work against the marking points and look for scientific key words.
Question 2 (2 marks)
Method
Why use graphite electrodes when investigating dissolved ions?
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Graphite is conducting and largely inert in the conditions used (1), helping avoid electrode material taking part in the reaction (1).
Question 3 (2 marks)
Analysis
How would you test a gas suspected to be hydrogen?
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Bring a lit splint to a small sample (1); it burns with a squeaky pop (1).
Question 4 (2 marks)
Evaluation
Suggest one improvement to make the investigation more reliable and explain why it helps.
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Repeat measurements and calculate a mean (1), reducing the influence of random variation/anomalies (1). Alternative valid improvement specific to the experiment with explanation can gain both marks.
This page was updated on: 16th June 2026