The addition of NaOH (sodium hydroxide) to a solution of cobalt sulfate (CoSO₄) causes a color change from pink to blue due to a chemical reaction that forms a complex ion. Here's a breakdown:

1. Initial Solution:

* Cobalt sulfate (CoSO₄) in solution exists as pink hydrated cobalt(II) ions ([Co(H₂O)₆]²⁺).

2. Adding NaOH:

* When you add NaOH, the hydroxide ions (OH⁻) from NaOH react with the hydrated cobalt(II) ions.

3. Complex Ion Formation:

* This reaction causes some of the water molecules surrounding the cobalt(II) ion to be replaced by hydroxide ions. This results in the formation of a blue tetrahedral complex ion: [Co(OH)₄]²⁻.

4. Color Change:

* The difference in color between the pink hydrated cobalt(II) ion and the blue tetrahedral complex ion is due to the change in the coordination environment around the cobalt atom. The different ligands (water vs. hydroxide) affect the way the cobalt atom absorbs and emits light.

In summary:

The addition of NaOH to cobalt sulfate solution causes a color change from pink to blue because it leads to the formation of a blue tetrahedral complex ion, [Co(OH)₄]²⁻, by replacing some of the water molecules surrounding the cobalt(II) ion with hydroxide ions.