The gas essential for combustion is oxygen. Combustion is a chemical reaction that occurs when a substance reacts rapidly with oxygen, releasing energy in the form of heat and light. This process is fundamental in many everyday applications, such as burning fuels for energy in cars, heating, and cooking. Oxygen's role is critical; without it, the combustion process cannot occur. When hydrocarbons like gasoline or wood burn, they combine with oxygen to produce carbon dioxide and water, along with energy. This reaction underpins various industries, including energy generation and manufacturing.