Aqueous means dissolved in water. Water supplies H⁺ and OH⁻ ions as well as the ions from the dissolved compound. Products can therefore differ from those of a molten compound. These GCSE rules use inert electrodes and a solution containing one ionic compound.
Cathode: if the metal is more reactive than hydrogen, hydrogen forms; otherwise the metal forms.
Anode: if halide ions (chloride, bromide, iodide) are present, the halogen forms; otherwise oxygen forms.
| Aqueous solution | Cathode (−) | Anode (+) |
|---|---|---|
| Sodium chloride | Hydrogen | Chlorine |
| Copper chloride | Copper | Chlorine |
| Sodium sulfate | Hydrogen | Oxygen |
| Copper sulfate | Copper | Oxygen |
Always check molten or aqueous before choosing products. In molten sodium chloride the cathode product is sodium; in its aqueous solution it is hydrogen.
Aqueous cathode: metal or hydrogen. Anode: halogen if halide present, otherwise oxygen.
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