The correct option for distinguishing between ethylene (ethene) and acetylene is **B. passing each gas through dilute potassium permanganate solution**. ### Explanation of the Correct Answer **Step 1: Understanding the Gases** - **Ethylene (C₂H₄)** is an alkene, which means it has a double bond between two carbon atoms. - **Acetylene (C₂H₂)** is an alkyne, characterized by a triple bond between two carbon atoms. **Step 2: Reaction with Potassium Permanganate** - Potassium permanganate (KMnO₄) is a strong oxidizing agent. When it comes into contact with alkenes and alkynes, it can react with them. - **Ethylene (C₂H₄)** will react with dilute potassium permanganate in a process known as oxidation. The double bond in ethylene allows it to react with KMnO₄, leading to the formation of a diol (a compound with two hydroxyl groups). This reaction will cause the purple color of the KMnO₄ solution to disappear, indicating a positive reaction. The reaction can be summarized as: \[ C_2H_4 + [O] \rightarrow C_2H_4(OH)_2 \] (where [O] represents the oxygen from KMnO₄) - **Acetylene (C₂H₂)**, on the other hand, does not react with dilute potassium permanganate under mild conditions. The triple bond is less reactive towards oxidation compared to the double bond in ethylene. Therefore, the purple color of the KMnO₄ solution will remain unchanged when acetylene is passed through it. ### Why the Other Options are Incorrect **A. Passing each gas through bromine water** - Both ethylene and acetylene can react with bromine water, leading to decolorization of the bromine solution. This is because both compounds can undergo electrophilic addition reactions with bromine due to their unsaturation (double or triple bonds). Therefore, this test cannot distinguish between the two gases. **C. Passing each gas through silver nitrate solution** - Silver nitrate does not react specifically with either ethylene or acetylene in a way that would allow for differentiation. While alkynes can form silver acetylide precipitates in the presence of silver nitrate, this reaction is not definitive for distinguishing between ethylene and acetylene, as it requires specific conditions and is not a straightforward test. **D. Treating each gas catalytically with excess hydrogen gas** - Both ethylene and acetylene can be hydrogenated to form ethane (C₂H₆). This means that treating either gas with hydrogen in the presence of a catalyst (like palladium or platinum) will convert both gases to the same product, making it impossible to distinguish between them using this method. ### Summary of Key Points - **Correct Method**: Use dilute potassium permanganate solution to distinguish between ethylene and acetylene. - **Ethylene** reacts with KMnO₄, leading to a color change (purple to colorless). - **Acetylene** does not react with KMnO₄ under mild conditions, so the color remains unchanged. - Other methods (bromine water, silver nitrate, hydrogenation) do not effectively differentiate between the two gases. This understanding of the reactivity of alkenes and alkynes with oxidizing agents is crucial for distinguishing between these two important hydrocarbons in organic chemistry.