Electrolysis of dilute sulfuric acid produces hydrogen and oxygen gases, which are collected as soap bubbles. When ignited, the bubbles explode with a loud “crack,” demonstrating electrolysis, gas recombination, and energy changes.

1. Set up a side-arm boiling tube with lead foil electrodes, a bung, and a delivery tube connected to a beaker of soapy water.
2. Fill the tube with sulfuric acid solution until just below the side-arm. Seal with the bung.
3. Place the delivery tube near (but not yet in) the soapy water.
4. Connect electrodes to a 12 V DC power supply with an ammeter, and switch on (1–2 A current).
5. Insert the delivery tube under the soapy water. Hydrogen forms at the cathode, oxygen at the anode, in a 2:1 volume ratio.
6. Wait until air in the tubing has been displaced by the hydrogen–oxygen mixture.
7. Use a spatula or teaspoon on a long stick to collect bubbles from the surface.
8. Ignite the bubbles with a match on the end of a pole; they explode with a sharp crack.

Exploding bubbles of hydrogen and oxygen - Royal Society of Chemistry:

Hydrogen Bubbles - Exploding Bubbles - SpanglerScienceTV:

📄 Exploding bubbles of hydrogen and oxygen - RSC: https://sciencequiz.net/ichemistry/wp-content/uploads/2019/02/Experiment-71-Exploding-bubbles-of-hydrogen-and-oxygen.pdf

• Use a Hoffman voltameter to collect and show the 2:1 gas ratio separately before igniting.
• Use graphite rods as the electrodes.
• Experiment with different voltages and concentration of sulfuric acid.
• Compare loudness of bubbles filled with pure hydrogen, pure oxygen, and the hydrogen–oxygen mixture.

• Wear safety goggles and ear protection.
• Perform only as a teacher demonstration — never scale up.
• Keep the Bunsen burner at least 1 m from the electrolysis setup.
• Lead foil is toxic — wash hands after handling and avoid ingestion.
• Sulfuric acid is corrosive — avoid contact with skin and eyes.
• Never ignite gas directly at the end of the tubing.
• Ensure the room is ventilated and that students keep a safe distance from the bubbles - it can be very loud.

• Why is twice as much hydrogen produced as oxygen? (Because water has formula H2O, so electrolysis gives a 2:1 volume ratio of H2:O2.)
• Why do the bubbles explode so violently compared with burning pure hydrogen? (Because the gases are mixed in the exact stoichiometric ratio, allowing all reactants to combust at once.)
• Why is sulfuric acid added to the water for electrolysis? (It increases conductivity by providing ions.)
• Why is this reaction considered both endothermic and exothermic? (Electrolysis requires energy to split water — endothermic; recombination releases energy explosively — exothermic.)
• How could hydrogen fuel be produced for large-scale energy use? (By water splitting, though efficient catalysts and energy sources are still being researched.)