### Correct Option: A. Pass each gas into lime water ### Detailed Explanation: To identify gases, we often use specific tests that can produce observable changes or reactions. In this case, we have five gases: sulfur dioxide (SO₂), carbon dioxide (CO₂), hydrogen (H₂), carbon monoxide (CO), and nitrogen monoxide (NO). Let's analyze the options provided to see which one can effectively identify at least one of these gases. #### Option A: Pass each gas into lime water - **Lime water** is a dilute solution of calcium hydroxide (Ca(OH)₂). When carbon dioxide (CO₂) is bubbled through lime water, it reacts to form calcium carbonate (CaCO₃), which is insoluble and causes the solution to turn cloudy. The reaction is as follows: \[ \text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 \downarrow + \text{H}_2\text{O} \] - **Sulfur dioxide (SO₂)** can also react with lime water, but it produces calcium sulfite (CaSO₃), which does not cause cloudiness. Therefore, while lime water can identify CO₂ effectively, it does not provide a clear indication for SO₂. - **Conclusion**: This test can definitively identify CO₂, making option A a valid choice for identifying at least one gas. #### Option B: Pass each gas into water and test with litmus paper - This method involves dissolving the gases in water and then testing the resulting solution with litmus paper to determine if it is acidic or basic. - **CO₂** dissolves in water to form carbonic acid (H₂CO₃), which is weakly acidic and would turn blue litmus paper red. - **SO₂** also dissolves in water to form sulfurous acid (H₂SO₃), which is acidic and would similarly turn blue litmus paper red. - **H₂** is neutral and would not change the color of litmus paper. - **CO** is also neutral and would not affect litmus paper. - **NO** can form nitric acid (HNO₃) in water, which is acidic and would turn blue litmus paper red. - **Conclusion**: While this method can identify CO₂, SO₂, and NO, it does not provide a clear distinction between them, making it less effective than option A. #### Option C: Pass each gas into concentrated sulfuric acid - Concentrated sulfuric acid (H₂SO₄) is a strong dehydrating agent and can react with some gases. - **H₂** would not react significantly with sulfuric acid. - **CO** does not react with sulfuric acid. - **NO** can react to form nitrogen dioxide (NO₂), but this does not provide a clear identification. - **SO₂** can react with sulfuric acid, but again, this does not yield a clear identification. - **CO₂** does not react with concentrated sulfuric acid in a way that would provide a clear identification. - **Conclusion**: This option is not effective for identifying any of the gases clearly. #### Option D: Expose each gas to atmospheric air - Exposing gases to atmospheric air does not provide a specific test for identification. - **H₂** is flammable and would burn in air, but this does not identify it specifically. - **CO** is also flammable and can produce carbon dioxide when burned, but again, this does not provide a clear identification. - **NO** can react with oxygen to form NO₂, but this does not help in identifying it. - **SO₂** and **CO₂** do not have distinctive reactions with air that would allow for identification. - **Conclusion**: This option is not effective for identifying any of the gases. ### Summary of Why Option A is Correct: - Option A provides a clear and definitive test for identifying carbon dioxide (CO₂) through the formation of a cloudy precipitate in lime water. - Other options either do not provide a clear identification or can identify multiple gases without distinguishing between them. ### Revision Summary: - **Lime water test**: Identifies CO₂ by turning cloudy. - **Litmus paper test**: Can indicate acidity but does not distinguish between gases effectively. - **Concentrated sulfuric acid**: Does not provide clear identification for any of the gases. - **Exposure to air**: Does not yield specific identification for the gases. By understanding these tests and their reactions, you can effectively identify gases in a laboratory setting.