To determine whether a solution contains chloride ions (Cl⁻), we can perform a series of tests that involve the addition of specific reagents. Let's analyze the options provided and identify the correct one, which is option C. ### Correct Option: C **C. A white precipitate when acidified with dilute HNO3 and then AgNO3 solution added.** #### Explanation of Why Option C is Correct: 1. **Acidification with Dilute HNO3**: - Chloride ions can be present in a solution along with other anions. To ensure that we are testing specifically for chloride ions, we first acidify the solution with dilute nitric acid (HNO3). This step is crucial because it removes any carbonate ions (CO₃²⁻) or sulfide ions (S²⁻) that could interfere with the test. Carbonates would produce carbon dioxide gas (CO₂) when acidified, and sulfides would produce hydrogen sulfide (H₂S), both of which could lead to misleading results. 2. **Addition of AgNO3**: - After acidification, we add silver nitrate (AgNO3) to the solution. Silver ions (Ag⁺) react with chloride ions (Cl⁻) to form silver chloride (AgCl), which is a white precipitate. The reaction can be represented as: \[ \text{Ag}^+ (aq) + \text{Cl}^- (aq) \rightarrow \text{AgCl} (s) \] - The formation of a white precipitate indicates the presence of chloride ions in the solution. ### Why the Other Options are Incorrect or Weaker: **A. A white precipitate on addition of AgNO3 and barium chloride solutions.** - **Why it's incorrect**: While AgNO3 does produce a white precipitate with chloride ions, the addition of barium chloride (BaCl2) does not contribute to the identification of chloride. Instead, it could lead to confusion as barium ions can also form precipitates with sulfate ions (BaSO4) or other anions. This option does not specify the necessary acidification step, which is critical for accurate identification. **B. A white precipitate when acidified with HCl, and then AgNO3 solution added.** - **Why it's incorrect**: Adding hydrochloric acid (HCl) to the solution before testing with AgNO3 is problematic because HCl itself contains chloride ions. This would lead to a false positive result, as the precipitate formed could be due to the HCl rather than the sample being tested. Therefore, this option is not valid for identifying chloride ions in an unknown sample. **D. A white precipitate when acidified with dilute H2SO4 and then barium nitrate solution added.** - **Why it's incorrect**: This option suggests using dilute sulfuric acid (H2SO4) and barium nitrate (Ba(NO3)2). While barium sulfate (BaSO4) does form a white precipitate with sulfate ions, this test does not specifically identify chloride ions. Instead, it tests for sulfate ions, making it irrelevant for the identification of chlorides. ### Summary of Key Points: - **Correct Test for Chlorides**: Acidify with dilute HNO3, then add AgNO3 to form a white precipitate of AgCl. - **Importance of Acidification**: Acidification removes interfering ions that could lead to false results. - **Avoiding Confusion**: Tests must be specific to the ion of interest; using HCl or other reagents that contain chloride can lead to misleading results. - **Understanding Precipitation Reactions**: Recognizing the specific precipitate formed (AgCl) is crucial for confirming the presence of chloride ions. This thorough understanding of the test for chlorides will help you confidently identify chloride ions in various solutions during your chemistry studies and exams.