The correct option is **D. C6H6 (Benzene)**. ### Explanation of the Correct Answer 1. **Understanding Hydrocarbons**: Hydrocarbons are organic compounds made up of hydrogen and carbon atoms. They can be classified into two main categories: aliphatic (which includes alkanes, alkenes, and alkynes) and aromatic hydrocarbons (which include compounds like benzene). 2. **Combustion of Hydrocarbons**: When hydrocarbons burn in the presence of oxygen (air), they undergo a combustion reaction. The general reaction for the combustion of a hydrocarbon can be represented as: \[ \text{C}_x\text{H}_y + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{Energy} \] The products of complete combustion are carbon dioxide (CO2) and water (H2O). However, if there is insufficient oxygen, incomplete combustion occurs, leading to the formation of carbon (soot) and carbon monoxide (CO). 3. **Sooty Flames**: A sooty flame is indicative of incomplete combustion, which occurs when there is not enough oxygen to fully oxidize the carbon in the hydrocarbon. Hydrocarbons with a higher carbon-to-hydrogen ratio tend to produce sooty flames because they are more likely to form solid carbon (soot) when burned. 4. **Analyzing the Options**: - **A. C2H6 (Ethane)**: Ethane is an alkane with a 2:6 (1:3) carbon-to-hydrogen ratio. It burns relatively cleanly and typically produces a non-sooty flame when sufficient oxygen is present. - **B. C3H8 (Propane)**: Propane is another alkane with a 3:8 (1:2.67) carbon-to-hydrogen ratio. Like ethane, it burns cleanly in sufficient oxygen, producing a non-sooty flame. - **C. C4H10 (Butane)**: Butane, also an alkane, has a 4:10 (1:2.5) carbon-to-hydrogen ratio. It burns cleanly under normal conditions, producing a non-sooty flame. - **D. C6H6 (Benzene)**: Benzene is an aromatic hydrocarbon with a 6:6 (1:1) carbon-to-hydrogen ratio. When benzene burns, it tends to produce a sooty flame due to its structure and the presence of a higher proportion of carbon relative to hydrogen. This leads to incomplete combustion, especially in limited oxygen conditions. ### Why Other Options Are Incorrect or Weaker - **A. C2H6 (Ethane)**: Ethane burns cleanly and does not produce soot under normal combustion conditions. Therefore, it does not fit the criteria for producing a sooty flame. - **B. C3H8 (Propane)**: Similar to ethane, propane also burns cleanly and does not produce soot, making it an unsuitable choice for this question. - **C. C4H10 (Butane)**: Butane, like ethane and propane, burns cleanly in sufficient oxygen and does not produce a sooty flame. Thus, it is not the correct answer. ### Summary of Key Points - Hydrocarbons can burn in air, producing either clean or sooty flames depending on their structure and the availability of oxygen. - A sooty flame indicates incomplete combustion, which is more likely with hydrocarbons that have a higher carbon-to-hydrogen ratio. - Benzene (C6H6) is the only option that produces a sooty flame due to its aromatic structure and carbon content. - Alkanes like ethane, propane, and butane burn cleanly and do not produce soot. ### Revision Summary - Hydrocarbons burn in air, producing CO2 and H2O in complete combustion. - Sooty flames indicate incomplete combustion, often due to a high carbon-to-hydrogen ratio. - Benzene (C6H6) produces a sooty flame, while alkanes (C2H6, C3H8, C4H10) burn cleanly. - Understanding the structure and combustion behavior of hydrocarbons is crucial for predicting flame characteristics.