Chemistry

Oxygen

Named by Lavoisier (1777) from Greek oxys (sharp/acid) + -genēs (producer): “acid-former”. He believed oxygen was the essential ingredient of all acids — wrong: e.g. hydrochloric acid (HCl) contains no oxygen. The element all acids actually share is hydrogen (acidity = donating H⁺ ions).

Electrolysis of water

Electrolysis of water releases the two gases at opposite electrodes:

• Cathode (negative, supplies electrons — reduction): 2H₂O + 2e⁻ → H₂ + 2OH⁻ — releases hydrogen.
• Anode (positive, takes electrons — oxidation): 2H₂O → O₂ + 4H⁺ + 4e⁻ — releases oxygen.

So adding electrons frees hydrogen, removing electrons frees oxygen; twice as much hydrogen as oxygen by volume (the 2:1 ratio in H₂O).