Required practical 9 · Chemistry · Chemistry Paper 1, both tiers

Electrolysis of aqueous solutions

Investigate what happens when aqueous solutions are electrolysed using inert electrodes, and develop a hypothesis.

The exam trap. Say what you do and what you see in a gas test. Hydrogen, not a reactive metal, forms at the negative electrode.

Variables

Independent (you change): The solution electrolysed (for example copper chloride, sodium chloride, copper sulfate)
Dependent (you measure): The product at each electrode
Control (you keep the same): Inert (graphite) electrodes
The same low voltage
The same concentration and volume of solution

Pour the solution into the beaker and put in the inert electrodes, not touching.
Why Inert electrodes do not react, so the products come from the solution.
Fill two small test tubes with the solution and invert one over each electrode.
Why This collects any gas produced.
Connect to the power supply at about 4 V and switch on.
Why A low voltage is safe and enough to electrolyse.
Look for a solid coating, bubbles or a colour change at each electrode.
Why These show which product forms.
Test gases: damp blue litmus is bleached by chlorine; a glowing splint relights in oxygen; a lighted splint gives a squeaky pop with hydrogen.
Why Each gas has its own test.
Switch off as soon as enough gas is collected, and repeat with the other solutions.
Why Chlorine is toxic, so only small amounts should be made.

Hazard	How to reduce the risk
Chlorine is toxic.	Use a fume cupboard or good ventilation and switch off promptly.
Copper compounds are harmful.	Wear eye protection and wash hands.

At the negative electrode the less reactive of the metal and hydrogen is produced.
At the positive electrode a halogen forms if a halide is present; otherwise oxygen forms.
Higher: 2Cl⁻ → Cl₂ + 2e⁻ and 4OH⁻ → O₂ + 2H₂O + 4e⁻.

Giving the result of a gas test without saying what you do.
Instead Describe the test and the result: damp litmus is bleached.
Predicting sodium at the negative electrode for sodium chloride solution.
Instead Sodium is more reactive than hydrogen, so hydrogen forms.
Predicting sulfur or sulfur dioxide from a sulfate.
Instead With no halide, oxygen forms at the positive electrode.