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.
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