Several different kinds of energy interact when a fuel is burned, or combusted, as in a campfire. A typical campfire uses wood, possibly with leaves, pine needles and cones, and other forest litter. This material is ignited, and then it continues to burn until the fuel is used up or the fire is put out.
Energy is released in the forms of heat, or thermal energy, and radiant energy that includes light—but where does this energy come from? Prior to burning the energy is stored in chemical bonds. It is called chemical bond energy or chemical potential energy.
It would be simplistic, however, to say the chemical potential energy is just sitting there waiting to be released. The fact is that some chemical bonds contain more energy than others. The products of combustion in the campfire, mostly carbon dioxide and water, contain chemical bonds that are particularly low in energy. The fuel, made up mainly of compounds of carbon, oxygen, and hydrogen in addition to the oxygen (O2) required for combustion, contain higher-energy bonds in general. It is the difference in energy between the bonds in fuel and oxygen and those in the combustion products carbon dioxide and water that is released as thermal and radiant energy. In other words, high-energy chemical bonds are replaced with lower-energy chemical bonds, leaving extra energy, which is released.