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Facts of the Matter Richard Brill |
Fire can both
rejuvenate and
destroy lifeWhen it's under control, fire is our best friend. Out of control, it is our worst enemy and invokes our worst fears. Fire is linked with life and is very important to it, not only because it is a carbon phenomenon, but also because it is important in recycling of nutrients and the process of growth, rejuvenation and evolution of forests and grasslands.
Fire was mankind's first technology, its only serious competitor for the title being the shaping of rock for tools. Since Prometheus allegedly stole it from the gods, it has been the most important technology as well as a source of infinite fascination. Even today, from carbon-fired electrical generators to carbon-burning automobile engines, fire in one form or another is the mainstay of our carbon-based civilization.
Virtually all combustible substances contain carbon and hydrogen, the same chemical elements that comprise the majority of atoms in our own bodies and, along with oxygen, are the primary constituents of all life past and present. Most carbon compounds fall into a category known as organic chemicals, so named because they were formerly thought, though incorrectly, to be capable of being produced only by biological processes.
At a deeper level, the properties of carbon-based substances are due to the chemical properties of the carbon atom. The carbon atom has four bonding sites that allow it to form chemical bonds with up to four different atoms.
Carbon's chemical cousin silicon shares the four bonding sites with carbon, but the carbon bonds are not as strong as those of silicon, especially bonds with oxygen, which is the most abundant atom in the silicate rocks that form the earth's crust. Silicon's preference to bond with oxygen is largely due to the relative sizes of the two atoms, but it gives silicon-based substances much different properties from those that are based on carbon. The silicon-oxygen bond is very strong and is stable up to very high temperatures, and is responsible for the high melting temperature of silicate rocks, which dominate the geological universe.
The biological world is ruled by carbon because the strength of the bonds is in the range of energies of visible and near visible light, and also in the range of thermal energies of molecules of water in the liquid state. This allows for radiant energy to be stored by the process of photosynthesis in plants, to be passed up the food chain to animals, and also allows for water's important role as an agent of transport of biochemicals.
One reason for carbon's suitability as the basis of life is that one or all of the bonding sites on a carbon atom may be with other carbon atoms, which in turn can bond with up to three other atoms, one or all of which can be carbon atoms, and so forth ad infinitum. This allows for long chains and branching molecules, and a virtually unlimited number of molecules of different shapes, sizes, structures and compositions. Unlike it's chemical kin silicon, which bonds almost exclusively with oxygen in nature, carbon forms bonds not only with oxygen, but also with hydrogen, nitrogen, sulfur and phosphorus, the other primary chemicals of life, often referred to simply as CHNOPS. Not coincidentally, these other CHNOPS atoms also combine readily with oxygen (except nitrogen) and form strong bonds (including nitrogen).
Oxygen is a ruthless thief and chemical home-wrecker. In addition to its role in forming silicate rocks, it comprises 20 percent of Earth's atmosphere and nearly 90 percent of the weight of water. It breaks chemical bonds and steals electrons from other atoms. It is such a powerful oxidant that in order to keep from being oxidized, all aerobic life forms, from bacteria to humans, have enzymes that control the oxidation. The process of organic decay is basically a biologically controlled oxidation, the atoms of the deceased organism combing slowly with oxygen to form molecules of carbon monoxide, carbon dioxide, water (dihydrogen oxide) and various oxides of sulfur and phosphorus.
Earth's atmosphere is necessary for aerobic life, but it is also highly corrosive and potentially explosive. A little more oxygen in the atmosphere, and many plants would spontaneously ignite; a little less, and many animals would not be able to breathe.
Oxidation may take place at various speeds, from imperceptibly slow, as in the decay of dry wood, to rapid, such as the browning of apples and potatoes that are exposed to the air, to violently rapid, such as in the explosion of gunpowder. Organic molecules that exist in contact with the atmosphere or in oxygen-rich waters are typically kept in a reduced state by action of enzymes that slow or prevent the oxidation.
Fire is the heat-driven, rapid and violent equivalent version of the oxidation process. At high temperatures the oxidation of compounds of lightweight CHNOPS atoms takes place as the atoms are energized by the heat beyond the strength of the carbon bonds to keep the organic molecules together.
The chemical reaction of combustion is a simple one that breaks the chemical bonds within a combustible substance and replaces them with oxygen. Like other chemical reactions, oxidation is faster at higher temperatures, but combustion requires a jump-start known as "activation energy" that we know as "kindling temperature." For example, a sheet of newsprint baked in the oven will turn brown gradually until it reaches a temperature of around 450 degrees Fahrenheit. Once it bursts into flames, the reaction will continue on its own, the energy released by combustion thereafter sustaining the reaction.
The products of fire are smoke, water vapor, various other gasses and ashes. Smoke consists of sooty carbon particles and oil droplets that did not find the oxygen with which to bond before they escaped the heat and cooled below the temperature threshold for their oxidation. Gasses such as ammonia and hydrogen cyanide are common but vary depending on the fuel. Smoke also contains fine particles of the oxides of other chemical elements, mostly metals such as potassium, which were included in the complex organic molecules or were present as salts in the fuel. Ashes contain the larger particles of oxides of metals such as potassium, sodium and calcium that were too heavy to be lifted by the fire's updraft.
In order for a fire to be sustained, three components are necessary. Pyrologists refer to this as the "fire triangle" of heat, fuel and oxygen. Extinguishing a fire always involves shutting off the supply of one or more of the three.
Water is the preferred substance for putting out fires because it is abundant, noncombustible and it both cools and smothers the fire. The chemical bonds between oxygen and hydrogen are among the strongest in nature, and they are stable at temperatures far above those produced in ordinary combustion, even at its most extreme. Water is also readily transportable, as a fluid its delivery can be controlled and it has a high specific and latent heat capacities that allow it to absorb large quantities of heat with small mass.
Forest fires, such as those raging through dry mountainous areas during the summer months, can create their own weather. The extreme heat, which can be upward of 1,600 degrees Fahrenheit, causes extreme updrafts, carrying smoke high into the atmosphere. Water vapor, which is a product of the combustion, condenses to form thunderclouds that can reach thousands of feet in height and produce lighting that further aggravates the fire. Small particles of soot and ash may be carried thousands of miles.
Although combustion of carbon-based substances adds carbon dioxide and water vapor to the atmosphere, both of which are greenhouse gasses, not all carbon dioxide is bad. There is a complex carbon cycle in nature that includes atmospheric carbon dioxide, dissolved carbon dioxide in both fresh and salt water, carbonate rocks and in the biosphere, us included.
The carbon cycle is closely linked to the oxygen cycle, and the balance between plant producers and animal consumers of carbon and oxygen regulate the levels of oxygen in the atmosphere. Forest fires are an important part of the carbon cycle and help to regulate the oxygen content of the atmosphere. Unfortunately, urban swell and increased recreational and commercial use of forests has produced conflict between man and nature.
Just as we should be aware and responsible with fire on the small scale, so should we be on a global scale, being as we are a small but increasingly significant repository of carbon in the earth's complex ecology, and dependent on it for maintaining an environment that supports and nourishes us.
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu
