Question

Which gas is evolved at the anode during electrolysis of water?

• A. Hydrogen
• B. Sulphur dioxide
• C. Nitrogen dioxide
• D. Oxygen

Answer 1

The correct option is D Oxygen

During its electrolysis, the water molecule (${\text{H}}_2\text{O}$) undergoes decomposition to form ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ ions.

The ${\text{H}}^{+}$ ions move towards the cathode (negatively charged electrode) and thus hydrogen gas evolves there. The reaction occuring at cathode is:

$2{\text{H}}_2\text{O (l)}+2{\text{e}}^{-}\to {\text{H}}_2\text{(g)}+2{\text{OH}}^{-}\text{(aq)}$

The ${\text{OH}}^{-}$ ions move towards the anode (positively charged electrode) and thus oxygen gas evolves there. The reaction occuring at anode is:

$2{\text{H}}_2\text{O (l)}\to {\text{O}}_2\text{(g)}+4{\text{H}}^{+}\text{(aq)}+4{\text{e}}^{-}$