See also: flame
Fire is the rapid oxidation of a combustible material releasing heat, light, and various reaction products such as carbon dioxide and water. If hot enough, the gases may become ionized to produce plasma. Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity might vary. Fire in its most common form can result in conflagration, which has the potential to cause physical damage through burning.
Chemistry
Main article: CombustionFires start when a flammable and/or a combustible material with an adequate supply of oxygen or another oxidizer (e.g. nitrates, nitrites, inorganic peroxides, permanganates)is subjected to enough heat and is able to sustain a chain reaction. This is commonly called the fire tetrahedron. Fire cannot exist without all of these elements being in place and in the right proportions(though as previously stated, another strong oxidizer can replace oxygen). For example, a flammable liquid will start burning only if the fuel and oxygen are in the right proportions.
Once ignited, a chain reaction must take place whereby fires can sustain their own heat by the further release of heat energy in the process of combustion and may propagate, provided there is a continuous supply of an oxidizer and fuel.
Fire can be extinguished by removing any one of the elements of the fire tetrahedron. Fire extinguishing by the application of water acts by removing heat from the fuel faster than combustion generates it. Application of carbon dioxide is intended primarily to starve the fire of oxygen. A forest fire may be fought by starting smaller fires in advance of the main blaze, to deprive it of fuel. Other gaseous fire suppression agents, such as halon or HFC-227, interfere with the chemical reaction itself.
Flame
Main article: FlameA flame is a mixture of reacting gases and solids emitting visible and infrared light, the frequency spectrum of which depends on the chemical composition of the burning material and intermediate reaction products. In many cases, such as the burning of organic matter, for example wood, or the incomplete combustion of gas, incandescent solid particles called soot produce the familiar red-orange glow of 'fire'. This light has a continuous spectrum. Complete combustion of gas has a dim blue color due to the emission of single-wavelength radiation from various electron transitions in the excited molecules formed in the flame. Usually oxygen is involved, but hydrogen burning in chlorine also produces a flame, producing hydrogen chloride (HCl). Other possible combinations producing flames, amongst many more, are fluorine and hydrogen, and hydrazine and nitrogen tetroxide.
The glow of a flame is complex. Black-body radiation is emitted from soot, gas, and fuel particles, though the soot particles are too small to behave like perfect blackbodies. There is also photon emission by de-excited atoms and molecules in the gases. Much of the radiation is emitted in the visible and infrared bands. The color depends on temperature for the black-body radiation, and on chemical makeup for the emission spectra. The dominant color in a flame changes with temperature. The photo of the forest fire is an excellent example of this variation. Near the ground, where most burning is occurring, the fire is white, the hottest color possible for organic material in general, or yellow. Above the yellow region, the color changes to orange, which is cooler, then red, which is cooler still. Above the red region, combustion no longer occurs, and the uncombusted carbon particles are visible as black smoke.
The National Aeronautics and Space Administration (NASA) of the United States has recently found that gravity plays a role. Modifying the gravity causes different flame types. The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, as in a candle in normal gravity conditions, making it yellow. In micro gravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient (although it may go out if not moved steadily, as the CO 2 from combustion does not disperse as readily in micro gravity, and tends to smother the flame). There are several possible explanations for this difference, of which the most likely is that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs. Experiments by NASA reveal that diffusion flames in micro gravity allow more soot to be completely oxidized after they are produced than diffusion flames on Earth, because of a series of mechanisms that behave differently in micro gravity when compared to normal gravity conditions. These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency.
In combustion engines, various steps are taken to eliminate a flame. The method depends mainly on whether the fuel is oil, wood, or a high-energy fuel such as jet fuel.
Typical temperatures of fires and flames
- Oxyhydrogen flame: 2000 °C or above (3645 °F)
- Bunsen burner flame: 1,300 to 1,600 °C (2,372 to 2,912 °F)
- Blowtorch flame: 1,300 °C (2,370 °F)
- Candle flame: 1,000 °C (1,830 °F)
- Smoldering cigarette:
- Temperature without drawing: side of the lit portion; 400 °C (752 °F); middle of the lit portion: 585 °C (1,085 °F)
- Temperature during drawing: middle of the lit portion: 700 °C (1,292 °F)
- Always hotter in the middle.
Temperatures of flames by appearance
The temperature of flames with carbon particles emitting light can be assessed by their color:
- Red
- Just visible: 525 °C (977 °F)
- Dull: 700 °C (1,292 °F)
- Cherry, dull: 800 °C (1,470 °F)
- Cherry, full: 900 °C (1,650 °F)
- Cherry, clear: 1,000 °C (1,830 °F)
- Orange
- Deep: 1,100 °C (2,010 °F)
- Clear: 1,200 °C (2,190 °F)
- White
- Whitish: 1,300 °C (2,370 °F)
- Bright: 1,400 °C (2,550 °F)
- Dazzling: 1,500 °C (2,730 °F)
Fossil record
Main article: Fossil record of fireThe fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period, 470 million years ago , permitting the accumulation of oxygen in the atmosphere as never before, as the new hordes of land plants pumped it out as a waste product. When this concentration rose above 13%, it permitted the possibility of wildfire. Wildfire is first recorded in the Late Silurian fossil record, 420 million years ago , by fossils of charcoalified plants. Apart from a controversial gap in the Late Devonian, charcoal is present ever since. The level of atmospheric oxygen is closely related to the prevalence of charcoal: clearly oxygen is the key factor in the abundance of wildfire. Fire also became more abundant when grasses radiated and became the dominant component of many ecosystems, around 6 to 7 million years ago ; this kindling provided tinder which allowed for the more rapid spread of fire. These widespread fires may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire.
Human control
Main article: Control of fire by early humansThe ability to control fire was a major change in the habits of early humans. Making fire to generate heat and light made it possible for people to cook food, increasing the variety and availability of nutrients. The heat produced would also help people stay warm in cold weather, enabling them to live in cooler climates. Fire also kept nocturnal predators at bay. Evidence of cooked food is found from 1.9 million years ago , although fire was probably not used in a co
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