The correct option for the question regarding why hydrogen is used in oxy-hydrogen flames for melting metals is **A. evolves a lot of heat when burnt**. ### Detailed Explanation: 1. **Understanding Oxy-Hydrogen Flames**: - An oxy-hydrogen flame is produced by burning hydrogen gas in the presence of oxygen. The reaction can be represented by the chemical equation: \[ 2H_2 + O_2 \rightarrow 2H_2O + \text{heat} \] - This reaction releases a significant amount of energy in the form of heat, which is essential for melting metals. 2. **Heat Evolution**: - The combustion of hydrogen is highly exothermic, meaning it releases a large amount of heat. The heat generated is sufficient to reach very high temperatures, often exceeding 3000 °C (5432 °F). This high temperature is crucial for melting metals, which typically require substantial energy to change from solid to liquid. 3. **Why Option A is Correct**: - Since the primary reason for using hydrogen in oxy-hydrogen flames is its ability to produce a large amount of heat when burned, option A is the most accurate choice. The heat produced is what allows for effective metalworking, such as welding and cutting. ### Analysis of Other Options: - **Option B: Combines explosively with oxygen**: - While it is true that hydrogen can form explosive mixtures with oxygen, this characteristic is not the reason it is used in oxy-hydrogen flames for melting metals. The explosive nature of hydrogen is more relevant in contexts like rocket propulsion or in safety discussions rather than in controlled applications like metal melting. - **Option C: Is a very light gas**: - Hydrogen is indeed the lightest gas, which allows it to rise quickly and disperse in the atmosphere. However, its lightness does not contribute to its effectiveness in melting metals. The melting process relies on the heat generated from combustion, not the physical properties of the gas itself. - **Option D: Is a rocket fuel**: - Hydrogen is used as a rocket fuel due to its high energy content and efficiency when burned with oxygen. However, this fact does not directly relate to its use in oxy-hydrogen flames for melting metals. The context of the question is specifically about metal melting, not propulsion. ### Common Pitfalls: - Students may confuse the properties of hydrogen (like being a light gas) with its practical applications. It’s important to focus on the specific context of the question. - Misunderstanding the nature of the combustion reaction can lead to incorrect conclusions about why hydrogen is used in certain applications. ### Revision Summary: - Oxy-hydrogen flames produce high temperatures necessary for melting metals. - The combustion of hydrogen is highly exothermic, releasing a lot of heat. - Other options (B, C, D) do not directly relate to the effectiveness of hydrogen in melting metals. - Focus on the heat evolution aspect when considering the applications of hydrogen in metalworking.