The correct option is **C. CO2**. ### Explanation of the Correct Answer (C. CO2) 1. **Understanding the Reaction**: When carbon dioxide (CO2) is bubbled through a solution of acidified silver nitrate (AgNO3), it reacts to form silver carbonate (Ag2CO3), which is a white precipitate. The reaction can be summarized as follows: \[ \text{CO}_2 + 2 \text{AgNO}_3 + \text{H}_2\text{O} \rightarrow \text{Ag}_2\text{CO}_3 \downarrow + 2 \text{HNO}_3 \] Here, the downward arrow (↓) indicates the formation of a precipitate. 2. **Formation of the Precipitate**: Silver carbonate (Ag2CO3) is insoluble in water, which is why it appears as a white precipitate when CO2 is present in the solution. This is a key characteristic of the reaction that helps identify CO2. ### Why the Other Options are Incorrect - **A. NH3 (Ammonia)**: - Ammonia does not react with acidified silver nitrate to form a white precipitate. Instead, it can form complex ions with silver ions, such as [Ag(NH3)2]+, which are soluble in water. Therefore, NH3 does not lead to the formation of a precipitate in this context. - **B. SO2 (Sulfur Dioxide)**: - Sulfur dioxide can react with silver nitrate, but it does not form a white precipitate. Instead, it can lead to the formation of silver sulfite (Ag2SO3), which is also soluble in water. Thus, SO2 does not produce a visible precipitate with acidified silver nitrate. - **D. HCl (Hydrochloric Acid)**: - Hydrochloric acid does not produce a precipitate with silver nitrate in the presence of acid. Instead, it forms silver chloride (AgCl), which is a white precipitate, but this reaction occurs when silver nitrate is mixed directly with hydrochloric acid, not when HCl is treated as a gas. In the context of the question, HCl as a gas does not lead to the formation of a precipitate with acidified silver nitrate. ### Summary of Key Points - **Correct Answer**: C. CO2 forms a white precipitate (Ag2CO3) with acidified silver nitrate. - **NH3**: Forms soluble complexes with silver ions, no precipitate. - **SO2**: Can react but does not form a precipitate with silver nitrate. - **HCl**: Forms AgCl precipitate but not in the context of the gas reacting with acidified silver nitrate. ### Revision Summary - **CO2 reacts with acidified silver nitrate to form a white precipitate (Ag2CO3)**. - **NH3 does not precipitate; it forms soluble complexes**. - **SO2 does not produce a precipitate with silver nitrate**. - **HCl forms a precipitate with silver nitrate but not as a gas**.