Apparatus used in the electrolysis of water. Image credit

Apparatus used in the electrolysis of water. Image credit

ZIMSEC O Level Combined Science Notes: Experiment: Electrolysis of water

Aim: To produce hydrogen gas using the electrolysis of water

Materials: beaker, test tubes, carbon electrodes, water, concentrated sulphuric acid, battery/8-12v power supply, connecting wires, splints, burner, variable resister


Electrolysis of water set up

Electrolysis of water set up

  1. Set up the apparatus as shown in the diagram above
  2. Add a few drops of concentrated sulphuric acid to the water and stir
  3. Allow the current to pass through the water. Adjust the current on the ammeter until there is a gentle evolution of gases at both electrodes
  4. Remove one test tube at a time and quick place your thumb over the mouth to prevent trapped gas from escaping
  5. Test the gas at the anode using a glowing splint and the gas at the cathode using a lighted splint

Results and Observations

  • Most of the gas is collected at the cathode (negative) electrode
  • This is because water is made up of two molecules of hydrogen and one molecule of oxygen so the gas at the cathode is roughly twice in volume when compared to the gas at the anode
  • The gas from the cathode burns with a popping sound and produces a blue flame
  • The gas from the other end (anode) relights a glowing splint


  • Water can be broken down using electrolysis
  • Sulphiric acid or potassium hydroxide can be used to improve the conductivity of the water
  • Oxygen is produced at the anode
  • Hydrogen is given off at the cathode
  • Hydrogen and oxygen are produced at a ratio of 2:1
  • This mirrors the chemical make up of water

To access more topics go to the Combined Science Notes page.