The blue color in a copper sulfate solution is due to the presence of the copper(II) ion (Cu²⁺).

Here's why:

* Copper sulfate (CuSO₄) is a salt that dissociates in water, forming copper(II) ions (Cu²⁺) and sulfate ions (SO₄²⁻).

* Copper(II) ions absorb certain wavelengths of light, primarily in the orange-red region of the visible spectrum. This absorption leaves the complementary blue wavelengths to be reflected back, resulting in the blue color we perceive.

The sulfate ions do not contribute significantly to the color of the solution.