When the cobalt (II) solution completely dries out, the color of the remaining solid changes to blue color. What would be the chemical formula of ion in the solid responsible for the color change?

Co²⁺ ions

Explanation:

The ion responsible for the color change in the cobalt solution is Co²⁺ ions. This is because the chemical formula of the given compound is:

CoCl₂;

of the two elements that combined to form this specie, Co is a transition element.

Transition elements are known to occupy the d-block on the periodic table. Most elements here are usually characterized by their variable oxidation states. They also produce a wide range of colors.