How It Works: Why Water Creates Competing Ions

⚙️ How It Works: Why Water Creates Competing Ions

When an ionic compound dissolves in water, it completely dissociates into its component ions. However, water itself also partially ionises: H₂O ⇌ H⁺ + OH⁻. This means that in any aqueous solution, there are always at least four types of ion present — the two from the dissolved salt, plus H⁺ and OH⁻ from water.

At the cathode, both the metal cation from the salt and the H⁺ ion from water arrive and compete to gain electrons. The H⁺ ion is more easily reduced (gains electrons more readily) than the ions of reactive metals such as sodium or potassium. This is because reactive metals have a very strong tendency to remain as positive ions — they gave up their electrons readily when forming ions in the first place, and they do not easily take them back. Hydrogen ions, being less "committed" to staying as ions, are preferentially discharged at the cathode whenever the metal is above hydrogen in the reactivity series.

At the anode, the competition is between halide ions (Cl⁻, Br⁻, I⁻) and hydroxide ions (OH⁻) from water. Halide ions are preferentially discharged because they are more easily oxidised — they give up electrons more readily than OH⁻ ions. If no halide is present, OH⁻ from water is oxidised instead, producing oxygen gas via: 4OH⁻ → O₂ + 2H₂O + 4e⁻.