Electrolysis of Water 2

Water is electrolyzed with an apparatus that allows the volumes of oxygen and hydrogen to be quantified.


  • Hoffman apparatus
  • voltage supply
  • platinum electrodes


  • water


  1. After introducing the Hoffman apparatus to the audience, the voltage supply is turned on to provide 2 V between the electrodes in the two burets.  Point out the formation of bubbles and the displacement of water at the tops of the inverted burets.
  2. After 5-10 miniutes, sufficient gas will be evolved that the volumes can be measured and compared for oxygen and hydrogen.
  3. If desired, the voltage can be turned off, and the hydrogen gas can be collected in an inverted test tube and ignited to give a small pop.  Likewise, a smoldering wooden stick can be placed above the oxygen buret and it will reignite to a flame when the O2 is released from the buret.


The overall water electrolysis reaction is:

2 H2O (l) → 2 H2 (g) + O2 (g)

The stoichiometry of this reaction shows that there are 2 moles of H2 gas formed for every 1 mole of O2 gas.  Assuming ideal gas behavior and identical water solubilities, there should be twice the volume of hydrogen formed as there was oxygen.






Special Safety Notes

  • the power supply is low voltage, but could still give a spark if not adjusted correctly.
  • hydrogen is a flammable gas, use care with open flames
  • oxygen is an accelerant, use care with open flames
Close up view of the tops of the burets after the electrolysis has occurred.
The complete apparatus and power supply.
Close up view of bubbles formed at the electrodes.