Question

A pink coloured salt turns blue on heating. The presence of which cation is most likely?
(A) $C{{u}^{2+}}$
(B) $F{{e}^{2+}}$
(C) $Z{{n}^{2+}}$
(D) $C{{o}^{2+}}$

Answer 1

Hint: As we know, most of the salts are coloured. The reason behind the colour of most of the salts is the presence of transition elements in it. The color of the transition metal ions is due to the presence of unpaired electrons in it and due to the electronic transition, colored light is emitted which is complementary to the color of the light absorbed.

Complete step by step solution:
- Among the given compounds $Z{{n}^{2+}}$ has a white coloured salt and $F{{e}^{2+}}$ salts are rarely pink in colour. $C{{u}^{2+}}$ salts have usually blue colour in the salt.
- $C{{o}^{2+}}$ Cobalt cation is a divalent metal cation and a monatomic dication. The salts of $C{{o}^{2+}}$ are pink in colour and the pink colour turns to blue on heating.
- Cobalt is in plus two oxidation states in the compounds such as hexaaqua cobalt (II) ion, hydrated cobalt (II) hydroxide, hexaamminecobalt (II) ion etc.
- An example of colour change of $C{{o}^{2+}}$ can be expressed by using the complex $CoCl{{ }_{2}}.6{{H}_{2}}O$
\[CoC{{l}_{2}}.6{{H}_{2}}O\xrightarrow{\Delta }CoC{{l}_{2}}.2{{H}_{2}}O\to Anhyd.CoC{{l}_{2}}\]
In this reaction the first complex is pink in colour, second one is purple in colour and the last complex which is anhydrous complex is blue in colour.
- From the above discussions it’s clear that the cation which changes its pink colour to blue is cobalt (II).

Therefore the answer is option (D) $C{{o}^{2+}}$.

Note: Keep in mind that the $C{{o}^{2+}}$is stable in aqueous solutions but in the presence of a complexion agent, $C{{o}^{2+}}$undergoes change in oxidation state from +2 to +3 and is easily gets oxidised. The reason is because Cobalt (III) has more tendency to form coordination complexes than cobalt in plus two oxidation states.