Bubbles of gas rise from the surface of the electrodes as water decomposes to H 2 and O 2 gas. Alternatively, an acid base indicator can indicate that a reaction has occurred. This example contains Universal Indicator, which turns green-blue in base, and pink in acid. Decompostion of water is a redox reaction. The oxidation reaction occurs at one electrode, and the reduction reaction at the other electrode.
Water is oxidized at the anode.
H 2 is produced. In the decompostion reaction, the volume of H 2 produced is twice the volume of O 2 .