Video Transcript
When a few drops of solution X are added to a sodium sulfite solution, a white
precipitate is formed, which turns black upon heating. What is the potential identity of solution X? (A) Sodium carbonate, Na2CO3; (B) silver nitrate, AgNO3; (C) silver chloride, AgCl;
(D) hydrochloric acid, HCl; or (E) silver sulfate, Ag2SO4.
In chemistry, we perform qualitative tests to reveal the identity of unknown
chemicals. The tests involve a series of reactions that yield very apparent indications in the
presence of specific compounds or ions. A color change, a new odor or gas, a temperature change, or the formation of a
precipitate can all be indicative observations. Our question asks us about one such test. We are asked about the identity of a certain compound that forms a white precipitate
in the presence of a sodium sulfite solution. Let’s have a look at the tests used to detect the presence of sulfite anions.
The primary test for sulfite anions is the addition of dilute hydrochloric acid. If our sample contains sulfite ions, such as a sodium sulfite solution, then the
sulfite ions will react and sulfur dioxide gas will be given off. Sulfur dioxide is a smelly, colorless acidic gas. If we wet filter paper with an oxidizing agent such as potassium dichromate that is
originally yellow, we can test for the identity of this gas. The filter paper will turn green when sulfur dioxide gas is oxidized.
The confirmatory test for the sulfite anion uses a silver nitrate solution. When silver nitrate is added to a solution that contains sulfite ions, a white
precipitate is formed. Upon heating, the white precipitate will turn black. This will only occur in the presence of sulfite ions, so we can use this test as a
confirmation of the presence of these anions. We now know that the given diagram is representing the confirmatory test for sulfite
anions involving silver nitrate. Since the known solution in our question is sodium sulfite, which contains sulfite
ions, we can deduce that solution X is silver nitrate.
Therefore, the correct answer is (B) silver nitrate, AgNO3.
