Question

The aqueous solutions of the following salts will be coloured in the case of:
(A) $Zn{{(N{{O}_{3}})}_{2}}$
(B) $LiN{{O}_{3}}$
(C) $Co{{(N{{O}_{3}})}_{2}}$
(D) $CrC{{l}_{3}}$
(E) Potash alum

Answer 1

Hint: The colour of the salt is a property of transition elements having unpaired electrons in the d-block which are responsible to show d-d transition which is one of the important properties of the d-block elements.

Complete Step by Step Solution:
d-d transition: The orbitals of d-block elements do not have the same energy and thus split into further energy states as per the complex formed. When the electrons show transition between these energy levels take place, the electrons absorb light and when these electrons reach back to the ground state radiating a wavelength in the visible region responsible for the appearance of colour in the aqueous solution.

So, among given options, we can say that options (B) and (E) are eliminated because the metals in the salt do not belong to the transition metal group and thus the options are eliminated.

In option (A), zinc metal exists in its +2 oxidation state i.e., there are ten electrons in the d-shell and no unpaired electrons are present in the d-shell. Thus, there will be no transition observed in the split d-orbitals, so no coloured solution will be formed.

For the complex $Co{{(N{{O}_{3}})}_{2}}$ and $CrC{{l}_{3}}$, the metal ions exist in $C{{o}^{+2}}$ and $C{{r}^{+3}}$ state which consist of unpaired d –electrons showing d-d transition and hence, show colour in the aqueous solution.
Thus, options (C) and (D) are correct.

Note: It is important to note that although zinc is placed under transition metal elements but due to its completely filled d orbitals, it does not show any colour and electronic transition in its ground state and common oxidation state and hence considered as pseudo transition metal.