What occurs during electrolysis of water?

• A. Water boils away
• B. Hydrogen peroxide forms
• C. Water splits into hydrogen and oxygen
• D. Water decomposes into hydrogen and nitrogen

Answer: (C)

Electrolysis of water uses electrical energy to break water molecules into their elements. When an electrolyte is added to water to improve conductivity, the reaction at the electrodes splits water into hydrogen gas at the cathode and oxygen gas at the anode. The half-reactions show that water is reduced to hydrogen gas and hydroxide, while water is oxidized to oxygen gas and hydrogen ions, and when combined the overall equation is 2 H2O → 2 H2 + O2. So you observe hydrogen and oxygen gas being produced, not boiling away, not forming hydrogen peroxide, and not producing nitrogen. The two gases typically come off in a roughly 2:1 volume ratio of hydrogen to oxygen.