# Question #b39e6

$C {u}^{2 +} \left(a q\right)$ has a lovely blue colour.
The anhydrous salt, $C u S {O}_{4}$, is pure white. It dissolves in water to form the hexa-aqua ion, ${\left[C u {\left(O {H}_{2}\right)}_{6}\right]}^{2 +}$, whose electronic transitions gives rise to the blue colour. The sulfate ion is along for the ride.