What Is Fire Made Of?

What is fire made of? You know that it generates heat and light, but have you ever wondered about its chemical composition or state of matter?

Chemical Composition of Fire

Fire is the result of a chemical reaction called combustion. At a certain point in the combustion reaction, called the ignition point, flames are produced. Ordinarily, flames consist primarily of carbon dioxide, water vapor, oxygen, and nitrogen.

In the usual combustion reaction, a carbon-based fuel burns in air (oxygen). Potentially, fire only contains gases from the fuel, carbon dioxide, water, nitrogen, and oxygen. However, incomplete combustion yields a host of other possibilities. Soot is a primary component of incomplete combustion. Soot mainly contains carbon, but various organic molecules may occur. Other gases found in fire include carbon monoxide and sometimes nitrogen oxides and sulfur oxides.

A candle flame consists of vaporized water, carbon dioxide, water, nitrogen, oxygen, soot hot enough that it is incandescent, and light/heat from the chemical reaction.

Fire Without Oxygen

However, fire does not actually require oxygen. Yes, the oxidizer most often encountered is oxygen, but other chemicals also work. For example, burning hydrogen with chlorine as an oxidizer also produces a flame. The product of the reaction is hydrogen chloride (HCl), so the fire consists of hydrogen, chlorine, HCl, light, and heat. Other combinations are hydrogen with fluorine and hydrazine with nitrogen tetroxide.

State of Matter of Fire

In a candle flame or small fire, most of the matter in a flame consists of hot gases. A very hot fire releases enough energy to ionize the gaseous atoms, forming the state of matter called plasma. Examples of flames that contain plasma include those produced by plasma torches and the thermite reaction.

The main differences between gases and plasma are the distance between particles and their electrical charge. Gases consists of molecules, atoms, and ions that are widely spaced. The distance between particles is much greater in plasma. Additionally, the particles in plasma are almost exclusively charged particles (ions).

Why Fire Is Hot

Fire emits heat and light because the chemical reaction that produces flames is exothermic. In other words, combustion releases more energy than is needed to ignite or sustain it. In order for combustion to occur and flames to form, three things must be present: fuel, oxygen, and energy (usually in the form of heat). Once energy starts the reaction, it continues so long as fuel and oxygen are present.

Cold Fire

While all fire produced heat or is exothermic, some fires are cooler than others. So-called cold fire refers to a fire that burns below a temperature of about 400 °C (752 °F). At this temperature, the flame of the fire is invisible, yet the reaction proceeds. While cold fire is fairly uncommon on Earth, scientists have produced it in space. In a microgravity environment, fire burns with a spherical flame. Cold fire burns differently from regular combustion. Normally, the heat of a fire (and gravity) push combustion products away from the reaction. In a cool flame, these products stay within range of the reaction and participate further. Ultimately, a cold fire can burn away its waste products.

On Earth, cooler flames come from certain volatile fuels. For example, alcohol produces a cooler flame than acetylene. The availability of oxygen also matters. When oxygen is limited, so is the reaction, making the fire cooler.

Sources

• Bowman, D. M. J. S.; et al. (2009). "Fire in the Earth system". Science. 324 (5926): 481–84. doi:10.1126/science.1163886
• Lackner, Maximilian; Winter, Franz; Agarwal, Avinash K., eds. (2010). Handbook of Combustion, 5 volume set. Wiley-VCH. ISBN 978-3-527-32449-1.
• Law, C.K. (2006). Combustion Physics. Cambridge, UK: Cambridge University Press. ISBN 9780521154215.
• Schmidt-Rohr, K. (2015). "Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O₂". J. Chem. Educ. 92 (12): 2094–99. doi:10.1021/acs.jchemed.5b00333
• Ward, Michael (March 2005). Fire Officer: Principles and Practice. Jones & Bartlett Learning. ISBN 9780763722470.