copper(I) ion is not stable in an aqueous solution.
Stability in aqueous medium depends on the hydration energy of ions when they attract to water molecules. ions forms stronger bonds releasing excess energy as they have higher charge density than ions. The additional energy required for the second ionization of copper is more compensated by the hydration so that ion looses an electron to form and Cu releasing this hydration energy. Hence copper(I) ion is not stable in an aqueous solution.
In an aqueous medium, Cu2+ is more stable than Cu+. This is because although energy is required to remove one electron from Cu+ to Cu2+, high hydration energy of Cu2+ compensates for it. Therefore, Cu+ ion in an aqueous solution is unstable. It disproportionates to give Cu2+and Cu.