### Correct Option: A. Pass each gas into lime water ### Explanation of the Correct Answer: Lime water is a dilute solution of calcium hydroxide (Ca(OH)₂). When certain gases are bubbled through lime water, they react with it to produce a noticeable change, specifically a precipitate. 1. **Sulfur Dioxide (SO₂)**: When SO₂ is passed into lime water, it reacts to form calcium sulfite (CaSO₃), which is slightly soluble in water. However, if the concentration of SO₂ is high enough, it can further react with oxygen in the air to form sulfuric acid (H₂SO₄), which will then react with calcium hydroxide to form calcium sulfate (CaSO₄), leading to a milky appearance in the lime water due to the formation of calcium carbonate (CaCO₃) when carbon dioxide is also present. \[ \text{SO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaSO}_3 + \text{H}_2\text{O} \] If the reaction continues with atmospheric oxygen: \[ \text{CaSO}_3 + \text{O}_2 \rightarrow \text{CaSO}_4 \] The milky appearance indicates the presence of SO₂. 2. **Carbon Dioxide (CO₂)**: CO₂ reacts with lime water to form calcium carbonate (CaCO₃), which is insoluble in water and causes the lime water to turn milky. \[ \text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \] 3. **Hydrogen (H₂)**: H₂ does not react with lime water, so it will not produce any change. 4. **Carbon Monoxide (CO)**: CO also does not react with lime water, so there will be no observable change. 5. **Nitric Oxide (NO)**: NO does not react with lime water either, so it will not produce any change. Thus, using lime water is effective for identifying SO₂ and CO₂, making option A the correct choice. ### Explanation of Why Other Options Are Incorrect: **B. Pass each gas into water and test with litmus paper**: - This method is not effective for identifying the gases listed. While CO₂ can dissolve in water to form carbonic acid (H₂CO₃), which would turn litmus paper red, the other gases (SO₂, H₂, CO, NO) do not produce a significant change in litmus paper. H₂ is neutral, CO is neutral, and NO does not affect litmus paper either. **C. Pass each gas into concentrated sulfuric acid**: - Concentrated sulfuric acid is a strong dehydrating agent and will react with some gases, but it does not provide a clear identification for the gases listed. For example, SO₂ can react with sulfuric acid, but it does not produce a distinct observable change that would allow for identification. H₂ and CO do not react significantly with sulfuric acid, and NO can be oxidized but does not yield a clear identification. **D. Expose each gas to atmospheric air**: - This method is not specific and does not provide a means of identification. While some gases may react with oxygen in the air (like NO), this does not lead to a clear identification of the gases. H₂ is flammable, CO is toxic, and SO₂ can react with oxygen, but none of these reactions provide a definitive test for identification. ### Summary of Key Points: - **Lime water** reacts with SO₂ and CO₂, producing a milky solution, making it a reliable test for these gases. - **Litmus paper** testing in water is ineffective for most gases listed, as only CO₂ would show a change. - **Concentrated sulfuric acid** does not provide clear identification for the gases. - **Exposing gases to air** does not yield specific identification results. This thorough understanding of the reactions and properties of the gases will help in identifying them accurately in practical scenarios.