Note about this article
This article refers to burns in chemistry. See also under burns (medicine).
Burns
In chemistry, a combustion usually refers to an oxidation in which heat, light, fire, and energy are released. For example, the alkane octane is an important component of gasoline:
- C8H18 (octane) + 12.5 O2 (oxygen) 8 CO2 (carbon dioxide) + 9 H2O (water)
In this reaction, the alkane is oxidized by the oxygen in the air. This produces the gas carbon dioxide and water vapor. The energy released drives, for example, cars with an internal combustion engine. It is a redox reaction in which octane acts as a reducing agent and oxygen as an oxidizing agent. We are familiar with various fuels from everyday life, for example wood and other plant materials, coal, diesel, methane gas (natural gas), ethanol or beeswax. All these materials have a very high carbon content. Substances in all aggregate states – liquids, solids and gases – are therefore combustible. Activation energy is required to start the reaction. Thus, energy must first be supplied to the wood with a kindling and an already burning match.
Combustion of compounds with other elements
The oxidation of other chemical elements and compounds is also referred to as combustion. For example, sulfur burns with a blue flame to form the toxic sulfur dioxide (SO2):
- S8 (sulfur) + 16 O2 (oxygen) 8 SO2 (sulfur dioxide)
Even metals can “burn”, for example elemental magnesium:
- 2 Mg: (elemental magnesium) + O2 (oxygen) 2 MgO (magnesium oxide)
Burning magnesium can not be extinguished with water, because magnesium reacts with water to form hydrogen, which is also flammable! The combustion of hydrogen produces water. This is the so-called oxyhydrogen reaction:
- 2 H2 (hydrogen) + O2 (oxygen) 2 H2O (water)
Also, the oxidant does not necessarily have to be oxygen – others exist such as chlorine gas or fluorine. “Silent burns” are oxidations that proceed slowly or unnoticed, such as the rusting of iron (formation of iron oxides) or energy production in the mitochondria.
Consequences for health and the environment
Burns often release gases and compounds with the smoke that are harmful to the human body and especially to the respiratory system. One important reason for this is that combustion often proceeds incompletely, i.e. carbon monoxide is formed instead of carbon dioxide, for example. Other compounds include nitrogen oxides, polycyclic aromatic hydrocarbons and aldehydes. An additional problem is the formation of fine dust. Finally, combustion can also have a negative impact on the environment, for example through the excessive formation of greenhouse gases such as carbon dioxide.
