Name the product at each electrode when these are electrolysed…

(a) Molten magnesium chloride. There is no water, so only $\text{Mg}^{2+}$ and $\text{Cl}^-$ are present. Magnesium forms at the cathode and chlorine at the anode.

(b) Aqueous zinc sulphate. At the cathode, $\text{Zn}^{2+}$ has $-0.76\ \text{V}$, which is above water's $-0.83\ \text{V}$, so zinc deposits. At the anode, sulphate is not oxidised, so water gives oxygen.

(c) Aqueous potassium iodide. At the cathode, $\text{K}^+$ is far below water, so hydrogen forms. At the anode, iodide has $0.54\ \text{V}$, below water's $1.60\ \text{V}$, so iodine forms. The solution around the cathode becomes alkaline.

(d) Aqueous silver nitrate, silver electrodes. Silver deposits at the cathode, because $0.80\ \text{V}$ is high. The silver anode is active and has a low potential, so the anode itself dissolves: $\text{Ag} \rightarrow \text{Ag}^+ + e^-$. Nothing is made from water.