A ligand exchange reaction involving chloride ions

If you add concentrated hydrochloric acid to a solution containing hexaaquacobalt(II) ions, the solution turns from its original pink color to a rich blue. The six water molecules are replaced by four chloride ions.

The reaction taking place is reversible.

If you add water to the blue solution, it returns to the pink color. This reaction is explored in more detail on the module addressing ligand exchange reactions.