Question

___________ turns acidified potassium permanganate from purple to colourless.
\[
  A.{\text{ }}C{O_2} \\
  B.{\text{ }}{H_2}O \\
  C.{\text{ }}C{l_2} \\
  D.{\text{ }}S{O_2} \\
 \]

Answer 1

Hint: In order to solve the problem we will first see the chemical formula of potassium permanganate and further, we will see the reason behind the purple colour of potassium permanganate. We will also see the reason why the compound changes its colour further on the basis of our understanding of this process; we will see the reaction involved in the colour change process of potassium permanganate from purple to colourless.

Complete step by step answer:
Given that potassium permanganate changes the colour from purple to colourless.
The chemical formula of potassium permanganate is $KMn{O_4}$ .
Potassium permanganate is made up from potassium ion and permanganate ion. The purple colour of the potassium permanganate is due to permanganate ion. Potassium permanganate acts a strong oxidizing agent and so it can be used to oxidize substances.
The colour change changes occur in oxidation and reduction reaction mostly.
In order to reduce potassium permanganate we need a strong reducing agent in order to reduce it and to change the colour from purple to colourless.
We know that sulphur dioxide $S{O_2}$ is a strong reducing agent.
When we bubble in the sulphur dioxide gas into the purple acidified potassium permanganate solution the manganate ion is reduced by sulphur dioxide as seen in the reaction below.
$2KMn{O_4}\left( {{\text{purple}}} \right) + 5S{O_2} + 2{H_2}O \to {K_2}S{O_4} + 2MnS{O_4}\left( {{\text{colourless}}} \right) + 2{H_2}S{O_4}$

From the above reaction and the understanding we can conclude.
Hence, $S{O_2}$ turns acidified potassium permanganate from purple to colourless.
So, the correct answer is “Option D”.

Note: In order to solve such types of problems students must remember some important characteristics of some common compounds and also the reaction involved between them. Students also must remember some common compounds which are good reducing and oxidizing agents.