Question Video: Determining a Reagent to Test for Phenol Chemistry

Video Transcript

Which of the following reagents could be used for the detection of phenol? (A) CaCl2, (B) HgSO4, (C) CuSO4, (D) KMnO4, (E) FeCl3.

Phenol, which has the chemical formula C6H5OH, is an organic molecule with a hydroxy or OH group bonded directly to an aromatic ring. There are two chemical tests that can be done to detect phenol in a solution. One test is to add bromine water to a solution thought to contain phenol. Bromine water has an orange color and contains diatomic bromine molecules. As the bromine molecules react with phenol, the bromine water gets decolorized and a white precipitate forms. However, when looking at the answer choices, we see that bromine water is not one of the reagents listed. So we will need to discuss another chemical test.

A second chemical test that can be done to detect phenol is to add a solution of iron(III) chloride. Iron(III) chloride is also known as ferric chloride, and the solution has a yellow-orange color. In the presence of phenol, the iron(III) chloride solution changes to a violet-purple color. This color is due to the formation of a phenol–iron complex. But the reaction is pretty complicated, so we won’t discuss it here.

Now, we need to determine the chemical formula of the iron(III) chloride reagent. The chemical formula of the iron(III) ion is Fe3+, and the chemical formula of the chloride ion is Cl1−. Therefore, the chemical formula of the compound iron(III) chloride must be FeCl3. In conclusion, the reagent that can be used for the detection of phenol is FeCl3, or answer choice (E).