The gas primarily responsible for combustion is oxygen. Combustion is a chemical reaction that occurs when a fuel, typically a hydrocarbon, reacts with oxygen to produce heat, light, carbon dioxide, and water vapor. In this process, oxygen acts as an oxidizing agent, allowing the fuel to undergo oxidation. The availability of oxygen is crucial for combustion, and it can vary in concentration in various environments, affecting the efficiency and speed of the combustion process. This reaction is fundamental in many applications, including engines, heating systems, and various industrial processes.