Question

Explain: A green solution of potassium manganate turns purple when $C{O_2}$ passes through the solution.

Answer 1

Transition metal shows various colors in different oxidation states. The color is responsible for the $d - d$ transition or charge transfer between the ligands and metal ions.

Complete step by step answer: Potassium manganate is an inorganic compound composed of potassium, manganese and oxygen. The chemical formula of potassium manganate is ${K_2}Mn{O_4}$.
When $C{O_2}$ is passed through water carbonic acid is generated. The carbonic acid undergoes dissociation and makes the solution acidic by releasing ${H^ + }$ ion in the solution.
Manganese is an element in the periodic table with atomic number $25$. It is a transition metal and the electron configuration of manganese is $\left[ {Ar} \right]3{d^5}4{s^2}$. Thus manganese can exhibit a highest oxidation state of $+ 7$.
Manganese is known to undergo a disproportionation reaction to produce other manganese compounds in acidic medium. The oxidation state of manganese in potassium manganate (${K_2}Mn{O_4}$ ) is $+ 6$ . Under the acidic conditions the manganate undergoes disproportionation to produce permanganate $+ 7$ oxidation state and manganese dioxide with $+ 4$ oxidation state.
The colour of potassium permanganate is purple and the colour of manganese dioxide is brown solid. Hence when $C{O_2}$ gas is passed through a green solution of potassium manganate the colour of the solution turns into purple due to the formation of potassium permanganate. The corresponding reaction is:
$3Mn{O_4}^{2 - } + 4{H^ + } \to 2Mn{O_4}^ - + Mn{O_2} + 2{H_2}O$
The colour of $Mn{O_4}^{2 - }$ ion is purple.

Note: The ability to exhibit such higher oxidation states are common with transition metals. All the transition metals possess inner d orbitals which they use to extend their valency and form multiple bonds which have different atoms or ions.