The chemical necessary for combustion is oxygen. Combustion is a chemical reaction that occurs when a fuel reacts rapidly with oxygen, producing heat and light. In most cases, the fuel can be organic, such as wood or fossil fuels, while oxygen is the oxidizing agent that initiates and sustains the burning process. This reaction releases energy in the form of heat, making it fundamental for various applications, including powering engines, heating, and generating electricity. Without sufficient oxygen, combustion cannot occur, highlighting its crucial role in this chemical process.