The correct option for distinguishing between ethylene (ethene) and acetylene is **A. passing each gas through bromine water**. ### Explanation of the Correct Answer **1. Understanding the Gases:** - **Ethylene (C₂H₄)**: This is an alkene, which means it has a double bond between the carbon atoms. It is unsaturated and can undergo addition reactions. - **Acetylene (C₂H₂)**: This is an alkyne, characterized by a triple bond between the carbon atoms. It is also unsaturated but has a higher degree of unsaturation compared to alkenes. **2. Reaction with Bromine Water:** - **Bromine Water Test**: Bromine water is a solution of bromine in water, which is typically orange-brown in color. When an unsaturated hydrocarbon (like ethylene or acetylene) is added to bromine water, it reacts with the bromine, leading to a color change. - **Ethylene Reaction**: Ethylene reacts with bromine water to form a dibromo compound, resulting in the decolorization of the bromine water. The reaction can be represented as: \[ C_2H_4 + Br_2 \rightarrow C_2H_4Br_2 \] - **Acetylene Reaction**: Acetylene also reacts with bromine water, leading to decolorization. The reaction is: \[ C_2H_2 + 2Br_2 \rightarrow C_2H_2Br_4 \] - **Distinguishing Feature**: Both gases will decolorize bromine water, but the rate and conditions of the reaction can differ. Ethylene reacts more readily than acetylene due to the presence of a double bond, which is more reactive than a triple bond in terms of addition reactions. ### Why the Other Options are Incorrect or Weaker **B. Passing each gas through dilute potassium permanganate solution:** - **Potassium Permanganate Test**: This solution is purple and can also be used to test for unsaturation. Both ethylene and acetylene will react with potassium permanganate, leading to a color change (decolorization). However, this test does not provide a clear distinction between the two gases, as both will react similarly, making it less effective for differentiation. **C. Passing each gas through silver nitrate solution:** - **Silver Nitrate Test**: This test is typically used to identify alkyl halides and does not apply to ethylene or acetylene directly. Neither gas will react with silver nitrate in a way that distinguishes them, making this option incorrect. **D. Treating each gas catalytically with excess hydrogen gas:** - **Hydrogenation Reaction**: Both ethylene and acetylene can be hydrogenated to form ethane (C₂H₆). This process does not distinguish between the two gases, as both will yield the same product when treated with hydrogen in the presence of a catalyst. Therefore, this option is also incorrect. ### Summary of Key Points - **Correct Test**: The best method to distinguish between ethylene and acetylene is by passing each gas through bromine water, which will lead to decolorization. - **Reactivity**: Ethylene reacts more readily than acetylene due to its double bond. - **Other Tests**: Potassium permanganate can react with both gases but does not provide a clear distinction; silver nitrate is not applicable, and hydrogenation does not differentiate between the two. - **Color Change**: The decolorization of bromine water is a clear visual indicator of unsaturation in hydrocarbons. This thorough understanding of the reactions and properties of ethylene and acetylene will help you effectively distinguish between these two important hydrocarbons in organic chemistry.