What color change is seen when nitrogen monoxide gas is passed through a solution of potassium permanganate? (A) Colorless to white, (B) colorless to yellow, (C) brown to colorless, (D) orange to green, or (E) purple to pale pink.
This question is asking about a portion of the test for nitrite ions. When we carry out this test on an unknown substance, we can determine whether or not nitrite ions are present. The first step of the test involves combining the mystery substance with hydrochloric acid. If nitrite ions are indeed present in the substance, they will react with the hydrogen ions from the hydrochloric acid to produce nitrous acid. However, nitrous acid is not very stable, so it readily decomposes into nitric acid, water, and nitrogen monoxide gas. Finally, we get to the reaction described in the question, which begins with the nitrogen monoxide gas produced in the previous reaction. We pass this nitrogen monoxide gas through a solution of potassium permanganate.
To simplify this equation, let’s just write the net ionic equation and leave out the spectator ion potassium. Although it doesn’t directly say it in the problem, in order for this reaction to take place, we need to acidify the potassium permanganate solution. We often do this with sulfuric acid, which will provide hydrogen ions. This reaction will produce nitrate ions, manganese ions, and liquid water. We can balance the equation with the coefficients as shown. The big change here is that the permanganate ions in the reactants lose oxygen atoms to become manganese ions as a product. Permanganate ions give the solution a rich purple color, while manganese ions give the solution a pale pink color. This color change corresponds to answer choice (E), purple to pale pink, the correct answer.
To review, when a substance with nitrite ions in it is exposed to hydrochloric acid, nitrogen monoxide gas is produced. When that gas is passed through a permanganate solution, the color changes from purple to pale pink. This color change indicates that the initial substance does indeed contain nitrite ions. So what color change is seen when nitrogen monoxide gas is passed through a solution of potassium permanganate? That’s choice (E) purple to pale pink.