Areaction occurs when a substance, usually containing carbon, reacts with oxygen to produce energy in the form of heat and light.

This is called a combustion reaction. A combustion reaction involves oxygen gas. Most combustion reaction are exothermic as they give off heat. When organic molecules (containing carbon and hydrogen) combust the reaction products are carbon dioxide and water as well as heat or light. Combustion reactions may be complete or incomplete, a complete combustion releases more energy than incomplete combustion.