Video Transcript
Which of the following gases is liberated when dilute hydrochloric acid is added to a solid sample that contains bicarbonate anions? (A) Steam, (B) hydrogen sulfide, (C) ammonia, (D) carbon dioxide, (E) sulfur dioxide.
Anions present in an unknown chemical substance can be identified using the process of qualitative analysis. The factor that determines whether an anion will react with the reagent used in a chemical test is the relative stability of the anion to the reagent. Hydrochloric acid is used first to test for any unknown anion. If the anion is not identified with dilute hydrochloric acid, then concentrated sulfuric acid is used next. Finally, if the anion could not be identified with sulfuric acid, then a solution of barium chloride is used. Dilute hydrochloric acid is the primary reagent used to test for the presence of bicarbonate anions.
This question is asking us to determine the identity of the gas that forms when dilute hydrochloric acid is added to a solid sample containing bicarbonate ions. During the reaction, the main observation made is of effervescence, which we notice visually as bubbles forming in the solution. Let’s take a look at the following net ionic equation to better understand what happens during this test. Bicarbonate ions react with the hydrogen ions present in the hydrochloric acid solution to produce carbon dioxide gas and liquid water.
One way to confirm that the gas produced is carbon dioxide is to bubble it through a solution of limewater. As the carbon dioxide gas comes into contact with the limewater, it becomes cloudy. It’s important to remember that the dilute hydrochloric acid test is the primary test for bicarbonate anions but cannot confirm the presence of bicarbonate anions. This is because carbonate ions also react with hydrochloric acid to produce carbon dioxide gas. So, to confirm that the sample contains bicarbonate ions, a second confirmatory test must be performed.
In summary, we’ve determined that the gas that is liberated when bicarbonate ions react with dilute hydrochloric acid is carbon dioxide, or answer choice (D).
