A certain 3d transition metal forms a +1 ion which is initially colourless in aqueous solution. However, the solution spontaneously turns blue over time due to a specific chemical process. Identify the metal and the primary process responsible for this observation.

Step-by-step solution

The +1 ion of copper ( Cu ⁺ ) has a 3d¹⁰ electronic configuration. Because its d-subshell is completely filled, it has no unpaired electrons and cannot undergo d-d transitions, making it initially colourless. In aqueous solution, the Cu ⁺ ion is highly unstable and spontaneously undergoes disproportionation to form Cu ²⁺ and solid copper ( Cu ). The reaction is: 2 Cu ⁺( aq ) Cu ²⁺( aq ) + Cu ( s ) . This process is thermodynamically driven by the highly negative hydration enthalpy of the Cu ²⁺ ion, which more than