Which of aqueous solution is coloured ? [IIT-1990]
Aqueous solutions of transition metal salts are often colored due to d-d electron transitions. $ Co (NO_3)_2 $ contains cobalt, a transition metal, which typically forms colored solutions. In contrast, $ Zn (NO_3)_2 $ and $ LiNO_3 $ contain zinc and lithium, respectively, which do not have partially filled d-orbitals and thus form colorless solutions. Potash Alum, while sometimes appearing colored due to impurities, is generally colorless in its pure form.