Video Transcript
When reacted with dilute aqueous sodium hydroxide and heated, the ammonium cation does not produce a precipitate. What product is formed? How could it be detected?
The reaction with dilute aqueous sodium hydroxide is a common test for cation identity in an unknown solution, as it often produces a distinctive precipitate with the cation in question and the hydroxide ions. However, ammonium cations do not form a precipitate when reacted with hydroxide anions. Instead, when the ammonium cation and hydroxide anions from sodium hydroxide react, ammonia gas and water are formed. When heating this reaction taking place, the ammonia gas can be observed bubbling through the solution. And the pungent odor of ammonia might also be observed.
However, bubbling is not a definitive enough way to detect the presence of a specific gas. And gases like ammonia are toxic, so odor is not a safe way to test its identity. A piece of moist litmus paper can be used to detect the presence of ammonia, as the ammonia gas reacts with the water on the litmus paper and forms an alkaline solution. We can tell this solution is alkaline because NH4OH dissociates into hydroxide ions. In the presence of an alkaline solution, the litmus paper will turn blue. Therefore, when the ammonium cation reacts with dilute sodium hydroxide and is heated, the product that is formed is ammonia. And it can be detected, as it turns moist litmus paper blue.
