Question

What is the visible evidence of a positive result for potassium permanganate test?

Answer 1

Generally, the visible evidence of a positive result of any indicator is the change in its color. Potassium permanganate acts as a strong oxidizing agent because we know that elements become more electronegative as the oxidation states of their atoms increase.

Complete answer:
In organic chemistry, $KMn{O_4}$ is used in qualitative analysis to test for the presence of unsaturation. The reagent is an alkaline solution of $KMn{O_4}$ . It is called Baeyer’s reagent after the name of the organic chemist Baeyer. $KMn{O_4}$ is composed of the potassium ion and the $MnO_4^ -$ ion. It is a purple crystalline salt which when dissolved in water gives an intense pink or purple solution. When potassium permanganate reacts with double bonds or triple bonds, it causes the color to change from pink to brown. Aldehydes and formic acid also give a positive test with potassium permanganate. Generally, when we add the strong oxidizing agent potassium permanganate to an alkene, the alkene is oxidized to a diol and the potassium permanganate is converted to brown $Mn{O_2}$ . Thus, if there is a color change from purple to brown in this reaction, it is a positive reaction. This shows that a positive result for potassium permanganate test is the disappearance of the purple color of potassium permanganate and the appearance of the brown suspension.

Note:
Remember that a positive Baeyer’s test or potassium permanganate test is indicated by a change in color from deep purple to brown. This is because the deep purple permanganate ion gets reduced to form a brown precipitate of $Mn{O_2}$. A negative result is a deep purple with no formation of precipitate.