Electrolysis of Water Demonstration

This classic demonstration illustrates the decomposition of water into its constituent elements. A small DC voltage is applied to a Hoffman apparatus.

2H2O(l) --> 2H2(g) + O2(g)

The apparatus consists of two gas collecting tubes positioned over platinum electrodes and a reservoir for containing the aqueous electrolyte solution. Hydrogen gas and oxygen gas bubbles form on the electrodes and rise to fill the tubes. 

Curriculum Notes 

This demo can be used at many different places in the curriculum. It is most often used at the beginning of a general chemistry course when the concept of chemical elements is being introduced. But it can also be used when the nature and strength of the covalent bond are being discussed or in a unit on electrochemistry.

Lead Time 
One day of lead time is required for this project.

The chemical equation for the electrolytic decomposition of water is:

2H2O(l) --> 2H2(g) + O2(g)

This demonstration, when viewed in light of Avogadro's hypothesis, illustrates that the elements which combine to form water do so in simple whole number ratios.

The half-reactions for this electrolysis reaction are

Anode:            2 H2O(l) –> O2(g) + 4 H+(aq) + 4e-

Cathode:         4 H2O(l) + 4e- –> 2H2(g) + 4 OH-(aq)

  • Hoffman apparatus filled with 0.1 M sulfuric acid electrolyte
  • low voltage DC power supply

Rotate the knob on the front of the power supply unit. Line up the pointer with the yellow line. Bubbles will immediately begin forming on the electrodes. When the apparatus has been running for about 10 to 15 minutes, compare the volumes of the gases that have been formed. There should be about twice as much hydrogen as oxygen.

Allow about 30 minutes for this demo, but you just start it and let it run.

Safety Precautions 

Dilute sulfuric acid is used as the electrolyte. Demonstrators should wear safety goggles whenever they approach the apparatus.



1. B.Z. Shakhashiri; Chemical Demonstrations: A Handbook for Teachers of Chemistry; Wisconsin 1992; Volume 4; p. 156

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group