Water Electrolysis

Split water into hydrogen and oxygen gases

Difficulty: Medium | Time: 15-30 minutes | Visual Impact: High

Historical Context

Water electrolysis was first performed in 1800, just weeks after Alessandro Volta announced his electric pile (battery). William Nicholson and Anthony Carlisle connected a Voltaic pile to water and observed gas bubbles - hydrogen at one electrode, oxygen at the other.

This experiment proved that water was a compound of hydrogen and oxygen (H₂O), not an element as Aristotle had taught. It helped establish the law of definite proportions and supported John Dalton’s atomic theory. The 2:1 volume ratio of hydrogen to oxygen provided direct evidence for the molecular formula.

Today, “green hydrogen” produced by electrolyzing water with renewable electricity is seen as a key technology for decarbonizing industry. The same reaction demonstrated in a classroom may help power the future.

Materials

  • Water - 500mL
  • Salt - 1 tablespoon (15g) OR baking soda - 2 tablespoons (30g)
  • 9V battery (or 6V DC power supply for faster results)
  • Two graphite electrodes from #2 pencils (expose ~3cm of graphite)
  • Two test tubes
  • Beaker (600mL)
  • Alligator clip wires

Procedure

  1. Dissolve electrolyte in 500mL water in beaker
  2. Fill two test tubes with solution, invert over electrodes
  3. Connect battery leads to electrodes with alligator clips
  4. Observe gas bubbles rising into tubes (visible within seconds)
  5. Cathode produces hydrogen (2× volume), anode produces oxygen (1× volume)

Reaction

\[\ce{2 H2O -> 2 H2 + O2}\]

At cathode (negative): \[\ce{4 H2O + 4 e^- -> 2 H2 + 4 OH^-}\]

At anode (positive): \[\ce{2 H2O -> O2 + 4 H^+ + 4 e^-}\]

The Science

Electrical energy splits water molecules: - Cathode (negative): Electrons reduce water, producing hydrogen gas - Anode (positive): Water is oxidized, producing oxygen gas - The 2:1 ratio of H₂:O₂ demonstrates the stoichiometry of water

The electrolyte (salt or baking soda) doesn’t participate in the reaction but makes the water conductive by providing ions to carry current.

Safety

Do NOT use salt with metal electrodes - produces chlorine gas! Use graphite only with salt. Baking soda is safer for all electrode types.

Hydrogen is flammable - avoid sparks near the collected gas.

Resources