Answer
$Co^{3+}$ is an acidic cation.
$Co(H_2{O)_6}^{3+} + H_2O(l) \lt -- \gt Co(H_2O)_5OH^{2+}(aq) + H_3O^+(aq)$
Work Step by Step
$Co^{3+}$ is considered an acidic cation, because it is a highly charged small** metal.
** It is on the first row of transition metals, so it is considered small.
1. Firstly, the cation formates a complex ion with the water molecules:
$Co(H_2{O)_6}^{3+}$
Then, it reacts with other water molecules, donating one proton, and producing $H_3O^+$.
