Electrolysis of Water

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Compounds, Electrochemistry, Elements and Periodic Table, Gases

Alternative titles: Splitting Water with Electricity

Summary

An electric current is passed through a sodium sulfate or sulfuric acid solution, splitting water into hydrogen and oxygen gases. The gases are collected and tested—hydrogen ignites with a flame, while oxygen relights a glowing splint.

Procedure

Fill the electrolysis apparatus with dilute sodium sulfate or sulfuric acid solution , ensuring both compartments are filled completely, then close the stopcocks.
Connect the apparatus to a direct current power supply set to about 10–12 volts.
Allow electrolysis to occur, collecting hydrogen gas at the cathode and oxygen gas at the anode.
Test the hydrogen by igniting it with a lit splint or bunsen burner.
Test the oxygen by inserting a glowing splint into the collected gas; it will reignite or glow more brightly.
Rinse the apparatus thoroughly after the demonstration to prevent corrosion of electrodes.

Links

Science at Home - SE1 - EP1: Electrolysis of Water - The Sci Guys (a much simpler setup):

Electrolysis of Water - Scott Milam:

📄 Electrolysis of Water - University of Washington: https://chem.washington.edu/lecture-demos/electrolysis-water

Variations

Add phenolphthalein indicator to the solution to show hydroxide ion formation at the cathode (solution turns pink).
Collect larger amounts of hydrogen and oxygen to combine in a test tube for a small “pop” explosion.

Safety Precautions

Safety goggles required.
Keep hydrogen away from open flames until intentionally tested.
Do not exceed 12 volts to avoid damaging equipment or blowing fuses.
Handle glassware and test tubes carefully to avoid breakage.
Rinse apparatus immediately after use to prevent electrode corrosion.

Questions to Consider

What are the products formed when water undergoes electrolysis? (Hydrogen and oxygen gases.)
Why is sodium sulfate added to the water? (It increases conductivity by providing ions but does not take part in the reaction.)
How can you identify oxygen gas in this experiment? (It relights a glowing splint.)
Why does hydrogen make a “pop” sound when ignited? (Because hydrogen combusts rapidly with oxygen to form water.)
How does this experiment demonstrate conservation of matter? (The atoms of hydrogen and oxygen in water are rearranged into new molecules without being lost.)