Video Transcript
Which of the following acidic radicals cannot be tested for using dilute hydrochloric acid? (A) Phosphate, PO4 3−; (B) sulfite, SO3 2−; (C) thiosulfate, S2O3 2−; (D) bicarbonate, HCO3−; or (E) carbonate, CO3 2−.
This is a question about testing for anions. In order to answer this question, we need to be familiar with the tests that we use to determine the presence or absence of various anions. Four out of these five anions can be tested for using dilute hydrochloric acid, which has a chemical formula HCl. Let’s look at what happens when we mix hydrochloric acid with some of the answer choices.
A solid substance with either carbonate ions or bicarbonate ions in it will combine with the hydrogen ions from the hydrochloric acid to produce water and carbon dioxide gas. The key product here is the carbon dioxide gas. That’s a sign that the starting anion was a carbonate ion or a bicarbonate ion. We can confirm that the gas produced is indeed carbon dioxide gas by using the limewater test. If we pass carbon dioxide through limewater, it will turn the limewater a milky cloudy white. There are further tests that we could use to distinguish between bicarbonate and carbonate ions. However, those tests are beyond the scope of this video. For now, we can simply say that both bicarbonate ions and carbonate ions can be tested for using dilute hydrochloric acid. So we can eliminate choice (D) and (E) from consideration.
When we react sulfite ions or thiosulfate ions with hydrochloric acid, we end up with two very similar reactions. Both ions react with the hydrogen ions from the acid to produce water and sulfur dioxide. The main difference is that thiosulfate ions produce sulfur as well. Solid sulfur is a yellow precipitate. So the presence of a yellow precipitate indicates that the initial ion was thiosulfate.
We can also test for the presence of sulfur dioxide gas. As an acidic gas, it will turn moist litmus paper red. So we can indeed test for sulfite and thiosulfate ions using hydrochloric acid. The presence of just an acidic gas indicates sulfite. The presence of an acidic gas in addition to a yellow precipitate indicates thiosulfate.
The last remaining answer choice, phosphate, cannot be tested for using dilute hydrochloric acid. Therefore, it is the correct answer.
To test for the presence of a phosphate ion, we add barium chloride. The phosphate ion and the barium ion combine to form barium phosphate. Barium phosphate is a white precipitate, although there are many white precipitates. So, to confirm that it’s barium phosphate, we can check if it dissolves in hydrochloric acid or if it turns yellow in a reaction with silver nitrate.
Choice (A), phosphate, is the correct answer, as it initially uses barium chloride instead of hydrochloric acid to test for the presence of that anion. So, which of the following acidic radicals cannot be tested for using dilute hydrochloric acid? That’s choice (A) phosphate, PO4 3−.
