An aqueous solution has more ions than a molten compound. As well as the ions from the dissolved compound, water supplies a few hydrogen ions (H+) and hydroxide ions (OH-). At each electrode two ions compete, and only one is discharged.
At the cathode, the metal forms only if it is less reactive than hydrogen. If the metal is more reactive than hydrogen, hydrogen gas forms instead. At the anode, if a halide ion (chloride, bromide or iodide) is present, the halogen forms. If there is no halide ion, hydroxide ions are discharged and oxygen forms.
Sodium chloride solution contains Na+, Cl-, H+ and OH-. Sodium is more reactive than hydrogen, so hydrogen forms at the cathode. Chloride ions form chlorine at the anode. The sodium ions and hydroxide ions are left behind, so the solution becomes sodium hydroxide, which is alkaline.
Copper sulfate solution contains Cu2+, SO42-, H+ and OH-. Copper is less reactive than hydrogen, so copper forms at the cathode. There is no halide, so oxygen forms at the anode. As copper ions are used up, the blue colour of the solution fades. Chlorine bleaches damp litmus paper, hydrogen gives a squeaky pop with a lit splint, and oxygen relights a glowing splint.
