Electrolysis of Water — Splitting the Molecule That Fooled Chemistry for Centuries

For most of recorded history, water was considered an element — one of the four classical elements of Aristotle, and a substance so fundamental that its decomposition seemed inconceivable. In 1800, just weeks after Alessandro Volta announced his invention of the voltaic pile (the first true battery), William Nicholson and Anthony Carlisle in London connected Volta's device to two wires dipping into water and watched in astonishment as streams of gas bubbles rose from both electrodes. They had split water into its components: hydrogen and oxygen.

The chemistry is deceptively simple: 2H₂O → 2H₂ + O₂. At the cathode (negative electrode), water molecules gain electrons and release hydrogen gas: 2H₂O + 2e⁻ → H₂↑ + 2OH⁻. At the anode (positive electrode), water molecules lose electrons and release oxygen gas: 2H₂O → O₂↑ + 4H⁺ + 4e⁻. The volume of hydrogen produced is exactly twice the volume of oxygen — reflecting the 2:1 ratio of hydrogen to oxygen atoms in each water molecule (H₂O).

Pure water conducts electricity poorly, so a small amount of electrolyte (sodium hydroxide or sulfuric acid) is added to carry the current. The electrolyte is not consumed — it merely provides ions to transport charge through the solution while the water itself is decomposed.

This single experiment — achievable with a battery, two electrodes, and a beaker of water — proved that water was a compound, launched the field of electrochemistry, and eventually led to industrial processes that produce millions of tonnes of hydrogen, chlorine, and aluminium by electrolysis every year.

SAFETY NOTE: Hydrogen gas is flammable and forms explosive mixtures with air. Do not perform near open flames. The quantities produced in this demonstration are small and safe in a ventilated space, but never collect hydrogen in sealed containers without proper venting.