What substance is burned in excess to produce an oxidizing flame?

An oxidizing flame is characterized by the presence of an excess of oxygen compared to the fuel being burned. This type of flame is hotter and produces a higher temperature than a neutral or reducing flame, which can be advantageous in various brazing and welding applications.

In the context of the options provided, oxygen is the substance that, when present in excess, enables the production of an oxidizing flame. By using more oxygen than fuel, the combustion process favors the formation of oxides, thereby creating the desired oxidizing environment. This flame is effective for certain types of materials and processes that require oxidation to occur, such as when working with metals that benefit from an oxidizing atmosphere.

The other substances listed—acetylene, hydrogen, and carbon dioxide—do not directly produce an oxidizing flame when burned in excess. Acetylene and hydrogen, when burned, can produce flame conditions that may not be primarily oxidizing, while carbon dioxide is not a fuel and would not contribute to the combustion process in the same way as the other fuels mentioned. Thus, oxygen stands out as the key ingredient for generating an oxidizing flame.