The correct option is **C. Na2S** (Sodium Sulfide). ### Explanation of the Correct Answer 1. **Reaction with Dilute Hydrochloric Acid**: - When sodium sulfide (Na2S) reacts with dilute hydrochloric acid (HCl), it produces hydrogen sulfide gas (H2S), which is known for its characteristic pungent smell, often described as similar to rotten eggs. - The reaction can be represented as: \[ \text{Na}_2\text{S} + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{S} \uparrow \] 2. **Decolorization of Acidified Potassium Tetraoxomanganate (VII) Solution**: - The acidified potassium tetraoxomanganate (VII) solution contains MnO4⁻ ions, which are purple in color. When hydrogen sulfide (H2S) is produced, it can reduce the MnO4⁻ ions to Mn²⁺ ions, which are colorless. - The reduction reaction can be summarized as: \[ \text{MnO}_4^- + 8\text{H}^+ + 5\text{e}^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O} \] - This reduction leads to the decolorization of the purple solution, confirming the presence of H2S. ### Why the Other Options are Incorrect - **A. Na2SO4 (Sodium Sulfate)**: - Sodium sulfate does not produce any gas when reacted with dilute hydrochloric acid. The reaction is: \[ \text{Na}_2\text{SO}_4 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{SO}_4 \] - No gas is evolved, and therefore, it cannot produce the pungent smell or decolorize the potassium tetraoxomanganate (VII) solution. - **B. Na2SO3 (Sodium Sulfite)**: - Sodium sulfite reacts with dilute hydrochloric acid to produce sulfur dioxide (SO2), which has a sharp, irritating smell, but it does not produce hydrogen sulfide (H2S). The reaction is: \[ \text{Na}_2\text{SO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{SO}_3 \] - The sulfurous acid (H2SO3) can decompose to produce SO2, but it does not have the same pungent smell as H2S and does not reduce MnO4⁻ effectively. - **D. Na2CO3 (Sodium Carbonate)**: - Sodium carbonate reacts with dilute hydrochloric acid to produce carbon dioxide (CO2), which is a colorless gas with no pungent smell. The reaction is: \[ \text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \uparrow \] - While CO2 can cause effervescence, it does not decolorize the potassium tetraoxomanganate (VII) solution. ### Summary of Key Points - **Correct Answer**: C. Na2S (produces H2S, a pungent gas). - **H2S reduces MnO4⁻ ions**, leading to decolorization of the solution. - **Na2SO4** does not produce gas; **Na2SO3** produces SO2, not H2S; **Na2CO3** produces CO2, which is not pungent. - Understanding the specific reactions and products is crucial for identifying the correct salt in such questions.