Fire - chemistry.

Fire - chemistry. I INTRODUCTION Fire, reaction involving fuel and oxygen that produces heat and light. Early humans used fire to warm themselves, cook food, and frighten away predators. Sitting around a fire may have helped unite and strengthen family groups and speed the evolution of early society. Fire enabled our human ancestors to travel out of warm, equatorial regions and, eventually, spread throughout the world. But fire also posed great risks and challenges to early people, including the threat of burns, the challenge of controlling fire, the greater challenge of starting a fire, and the threat of wildfires. As early civilizations developed, people discovered more uses for fire. They used fire to provide light, to make better tools, and as a weapon in times of war. Early religions often included fire as a part of their rituals, reflecting its importance to society. Early myths focused on fire's power. One such myth related the story of Vesta, the Roman goddess of the hearth. To honor Vesta, the high priest of the Roman religion periodically chose six priestesses, called Vestal Virgins, to keep a fire going in a community hearth. Keeping a controlled fire burning played a central part in communal life. Before the invention of modern implements, starting a fire, especially in adverse weather, usually required much time and labor to generate sufficient friction to ignite kindling. If people let their fire go out, they had to spend considerable time to start it again before they could eat and get warm. Today people naturally focus not on starting fires but on using fire productively and on preventing or extinguishing unwanted fires. We use fire to cook food and to heat our homes. Industries use fire to fuel power plants that produce electricity. At the same time, fire remains a potentially destructive force in people's lives. Natural fires started by lightning and volcanoes destroy wildlife and landscapes. Careless disposal of cigarettes and matches or carelessness with campfires leads to many wildfires. Fires in the home and workplace damage property and cause injury and death. Fires usually cost the United States and Canada more each year than floods, tornadoes, and other natural disasters combined. Scientists and fire protection engineers work together to help people use fire safely and productively. Smoke detectors and automatic sprinklers in homes and the workplace have reduced property loss, deaths, and injuries due to fire. Engineers continue to develop more fire-resistant materials for use in furniture, buildings, automobiles, subway cars, and ships. The development of new engineering approaches and new building codes and standards has led to safer buildings without dramatically increasing costs of construction. II EARLY USE OF FIRE The earliest use of fire by humans may have occurred as early as 1.4 million years ago. Evidence for this was found in Kenya--a mound of burned clay near animal bones and crude stone tools, suggesting a possible human campsite. However, this fire could have resulted from natural causes. Homo erectus, a species of human who lived from about 1.8 million to about 30,000 years ago, was the first to use fire on a regular basis. Evidence of a fire tended continuously by many generations of Homo erectus, dating to about 460,000 years ago, has been found in China. Scientists have also found evidence of tended hearths dating back as many as 400,000 years in several parts of France. Homo erectus was the first human species to leave equatorial Africa in large numbers and spread to other continents. Many scientists believe that the use of fire enabled Homo erectus to adapt to new environments by providing light, heat, and protection from dangerous animals. Tending fires probably helped foster social behavior by bringing early humans together into a small area. Fires may have tightened family groups as the families congregated around a fire to protect their young. Homo erectus may have used fire to cook food. The use of fire became widespread throughout Africa and Asia about 100,000 years ago. By this time anatomically modern humans, Homo sapiens, had evolved and existed alongside their near relatives, the Neandertals (Homo neanderthalensis). Clear indications of hearths have been found in Israel in Neandertal settlements that date from 60,000 years ago. The Neandertals died out about 28,000 years ago. III EARLY FIRE-MAKING TECHNIQUES Sometime after people began to use stone for tools, they found that by rubbing together pieces of flint they could produce sparks that would set fire to wood shavings. Scientists have found evidence that people used pieces of flint and iron to produce sparks for fires 25,000 to 35,000 years ago. Early people also learned to make fires by rubbing together pieces of wood until the wood produced a hot powder that could light kindling. Later, people made fires by using wood devices that had been developed for other purposes. The fire drill was an adaptation of the bow and the drill. It consisted of a block of wood and a stick that was fixed in the looped string of a small, curved bow. The fire builder moved the bow in a sawing motion, with one end of the stick against the block of wood. This motion rotated the stick rapidly against the wood block, creating friction between the end of the stick and the block of wood. The friction produced a glowing wood powder that could be fanned into a flame and used to light a fire. Early people of southeastern Asia produced fire another way. They used a wood piston to compress air inside a bamboo tube that contained wood shavings. The compressed air became increasingly hotter, eventually igniting the shavings. The people of ancient civilizations improved on methods of fire-making. Glassmaking among the Greeks led to the development of lenses, which the Greeks used to focus sunlight on, and thereby ignite, bundles of dry sticks. As the use of metals in toolmaking increased, people developed the tinderbox. This moisture-proof, metal carrying case held tinder, usually charred cotton or linen cloth, and pieces of steel and flint. Striking the steel and flint together produced a spark that lighted the tinder. Later the Japanese devised a tinderbox that operated like a present-day cigarette lighter, in which the rotary motion of a metal wheel against flint set off sparks in tinder. Finally, in the mid-19th century, a reliable form of the phosphorus match was developed. IV FIRE AND THE ADVANCE OF CIVILIZATION As early people began to live in larger communities and to develop more advanced technologies, fire became a central part of their lives. Fire continues to be essential to humans today, although its presence may be hidden in gas-fired ovens and furnaces and thus less noticeable than before. A Prehistoric Uses of Fire Thousands of years ago hunter-gatherers (people who lived by hunting and gathering wild food) developed a number of valuable uses for fire. With fire they could remain active after the sun set, protect themselves from predators, warm themselves, cook, and make better tools. People began using fire as a source of light by taking advantage of the glow of wood-burning fires to continue their activities after dark and inside their dwellings, which were usually natural caves. Eventually people learned to dip branches in pitch to form torches. They created crude lamps by filling a hollowed out piece of stone with moss soaked in oil or tallow (a substance derived from animal fat). By cooking with fire, prehistoric people made the meat of the animals they killed more palatable and digestible. They learned to preserve meat by smoking it over a fire, vastly decreasing the danger of periodic starvation. Cooking also enabled them to add some formerly inedible plants to their food supplies. Fire enabled people to make better weapons and tools. In prehistoric times, hunters formed spears from tree branches by burning the tips of the branches and then scraping the charred ends into a point. They used fire to straighten and harden tools made of green wood. People eventually learned to control the spread of a fire by blowing at it through reed pipes. They then used this technique to burn hollows in logs to create cradles, bowls, and canoes. B Fire in Early Civilizations When prehistoric people developed the ability to cultivate crops and raise animals, they began to form permanent communities. These communities amassed food surpluses, enabling some people to devote their time to becoming skilled artisans. The artisans first used fire to make pottery and bricks. The first potters worked around 6500 BC in Mesopotamia, one of the earliest centers of civilization, located in modern-day Iraq and eastern Syria. They placed wet clay vessels in open fires to harden and waterproof them. By 3000 BC, Egyptian potters used fire in earthen kilns, or ovens, to bake bricks out of a mixture of mud and straw. Later, potters in Babylonia and Assyria, in the area now known as Iraq, used fire in stone kilns to create high temperatures that produced extremely durable pottery. Fire became the center of daily life in the ancient civilizations. Most of the mud, thatch, or wood houses in which ancient people lived contained a hearth, or fireplace, in the center. Smoke escaped through a hole directly overhead in the roof. Some of the houses, as well as tenements in crowded cities such as Rome and Athens, were heated by braziers (metal pans that held charcoal fires). The large houses of the rich in the Roman Empire were heated by movable stoves, or even furnaces, from which hot air flowed to a heat chamber under some of the rooms. Modern household stoves and furnaces stem from these developments. Ancient peoples developed improved devices for using fire to provide light. By 2000 BC they began using candles made of yarn or dry rushes dipped in animal fat. The Egyptians and Greeks introduced more advanced forms of the oil lamp, filling a shell or carved stone with animal or vegetable oil and introducing a floating wick. Later people began to use pottery or metal dishes with a spout for the wick. Lamps remained the basic source of light, with gas and kerosene later being used as fuel, until the development of the electric bulb in the 19th century. Fire was essential in metalworking, which developed after 4000 BC. At this time Sumerian artisans, who preceded the Babylonians, melted copper ore for casting tools and weapons in a fire over an earthen hearth. The hearth contained a hole to collect the hot, liquid metal. Later, artisans lined the hearth hole with stone, creating the first furnace. Eventually, to increase the heat, they used bellows to force air into the fire and developed the first blast furnace. People also found they could create a hotter fire by burning carbonized (partially burned) sticks and twigs. They eventually produced charcoal, a compact, efficient fuel, by slowly smoldering wood in an oven with little air. The history of people's use of fire includes many difficulties involved in controlling fire. Early cities were often ravaged by fires. The ancient city of Troy, located in present-day Turkey, was destroyed several times by fire, perhaps due to war, perhaps to accident. One of the world's greatest losses was caused by a fire in the great library in Alexandria, Egypt, in 48 C BC. This fire destroyed the world's most complete collection of ancient Greek and Roman writings. Modern Uses of Fire Fire continues to be a basic, everyday element of most people's lives. Any home appliance that uses methane, propane, or oil relies on fire to operate. These appliances include gas- or oil-fired (but not electrically operated) water heaters, boilers, hot air furnaces, clothes dryers, stoves, and ovens. Many people use wood or, sometimes, coal in fireplaces or stoves to supplement the main heating system in their homes. In the countryside, people destroy leaves and brush by burning them. People also make outdoor fires to cook food in barbecues and over campfires. Today, many people enjoy sitting around a campfire, keeping warm and telling stories, just as people have for tens of thousands of years. Industries use fire to manufacture products and dispose of waste. Companies use heating and drying appliances similar to, but often much larger than, the ones in homes. Large industrial incinerators destroy garbage, including household, medical, and industrial waste. Fire can render toxic waste harmless when it burns such waste in special incinerators. This waste often cannot be destroyed in any other way. Fires also heat large boilers to generate steam, which then powers large turbines. These turbines generate electricity that provides power and heat to industries and homes. Large power plants may generate electricity using fuels such as coal, gas, and even wood or garbage to create fires. In some parts of the world, people use fire to prepare land for growing crops. Farmers in developed countries may burn plant material after a harvest to clear fields and return nutrients to the soil. Small-scale farmers in tropical regions sometimes practice slash-and-burn agriculture, in which wild plants and trees are burned to clear patches of land for cultivation and to quickly enrich nutrient-poor tropical soils. In recent decades widespread use of slash-and-burn agriculture has caused significant damage to the world's rainforests. People use fire as a weapon in times of war. Armies use napalm, a highly flammable substance, to spread fire. The fire can either directly kill enemy soldiers or destroy foliage, making enemy soldiers easier to find. V CHEMISTRY OF FIRE Fire results from a rapid chemical reaction between a fuel, such as wood or gasoline, and oxygen. Reactions that involve oxygen and other elements are called oxidation reactions. Chemists use the word combustion to refer to the oxidation reaction that produces fire. Combustion generates light, heat, gases, and soot. A How Combustion Occurs Several important factors need to be present for combustion to occur. The first requirements are fuel and oxygen. Fuel for a fire may range from trees in a forest to furniture in a home to gasoline in an automobile. The oxygen in the reaction usually comes from the surrounding air. The next requirement for combustion is an initiating energy source, or source of ignition. Ignition sources may be in the form of a spark, a flame, or even a very hot object. The ignition source must provide enough energy to start the chemical reaction. Finally, a chemical chain reaction (reaction that continuously fuels itself) must occur between the fuel and oxygen for combustion to take place. A1 Fuel and Oxygen Most combustible fuels begin as solids, such as wood, wax, and plastic. Many fuels that people burn for energy, including gasoline and methane (natural gas), begin as either a liquid or a gas. Any fuel must be in a gaseous state (so that it can react with oxygen) before a fire can occur. Heat from the fire's ignition source, and later from the fire itself, decomposes solid and liquid fuels, releasing flammable gases called volatiles. Some solids, such as the wax in a candle, melt into a liquid first. The liquid then evaporates, giving off volatiles that may then burn. Other solids, such as wood and cotton, decompose and evaporate directly. In a wood fire, gases given off by the decomposing wood enter the flame, combine with oxygen from the surrounding air, and ignite. The heat from the flame decomposes more wood, thus adding more flammable gases to the flame and creating a self-supporting process. Most common fuels consist of compounds containing the elements carbon and hydrogen. Fuels often also contain oxygen, nitrogen, chlorine, and sulfur. Cellulose is the principle combustible compound in wood, paper, and cotton. It contains carbon, hydrogen, and oxygen. Plastics that burn, such as polyvinylchloride (PVC), polystyrene, polymethyl methacrylate (PMMA), nylon, and polyurethane, are composed mostly of carbon and hydrogen. Liquid fuels include oil and gasoline, while gaseous fuels include methane, propane, and hydrogen. All of these fuels (except pure hydrogen) contain both carbon and hydrogen. A2 Ignition Source A fire can start when a fuel becomes so hot that it releases sufficient flammable gases for combustion to occur. At this temperature, called the fuel's piloted ignition temperature, a spark or flame will start the combustion reaction. One source of piloted ignition is an open flame, such as that from a match or lighter. Sparks, such as those generated by electricity, may also ignite a fire. Engineers and scientists usually use the term piloted ignition to refer to solid fuels. Liquid fuels have, instead, a flash point temperature. At a liquid's flash point, an ignition source will cause a flame to flash across the surface of the liquid. The unpiloted ignition temperature of a fuel, also called its spontaneous ignition temperature, is the temperature the fuel must reach to ignite on its own. It is higher than the piloted ignition temperature, because a flame or spark is not present to provide the extra heat needed to kick-start the chemical reaction. Heat within the fuel provides this energy. Some fuels do not have a spontaneous ignition temperature because they break down into other substances before they can ignite on their own. Flammable gases have just one ignition temperature. They will ignite at this temperature if they are present in the right concentrations. Ignition depends not only on a fuel's ignition temperature but also on the way the fuel absorbs heat. This absorption determines how heat will affect the fuel's temperature. A fuel's capacity to absorb heat depends on the type of fuel involved and its arrangement. Thick logs, for example, can absorb a large amount of heat before they reach their ignition temperature. Small twigs, however, need just a small amount of heat to reach the same ignition temperature. Fuels also need to absorb heat at or above a certain rate for ignition to occur. (The absorption rate can be expressed as units of heat absorbed per unit of time.) At the minimum absorption rate, the fuel will eventually reach its ignition temperature. A piece of wood will never ignite if the ignition source produces heat at a rate slower than the minimum rate required for ignition. A3 Chain Reaction The final requirement for a fire is a chemical chain reaction. The heat of the ignition source starts the reaction, and heat from the fire's flame continues the reaction. The flame needs to heat the fuel and make it release enough flammable gases to continuously support the chemical reaction. A common example of combustion is the burning of wood. When an ignition source heats wood to a sufficient temperature, about 260°C (500°F), the cellulose in the wood decomposes, producing volatile gases and char. The average composition of the gases can be represented by the compound CH2O, where C stands for carbon, H stands for hydrogen, and O stands for oxygen. Under ideal conditions, CH2O reacts with oxygen in the air and produces carbon dioxide (CO2) and water vapor (H2O). In the real world conditions are not ideal, so fires often produce other products as well, such as carbon monoxide (CO) and soot. The following equations show the two main stages involved in burning wood. The italicized letters represent numbers that depend on the conditions of the fire, such as how quickly the fire burns and the specific composition of the wood. B Burning Rate Different kinds of fires burn at different rates--one fire may slowly smolder, while another may quickly use up its fuel. The rate at which a fire burns depends on the composition of the fuel, the surface area of the fuel, and the amount of oxygen that is available. Most plastics burn at twice the rate of cellulose fuels, such as wood and leaves, because of the different chemical reactions involved. The burning rate of the same fuel, however, can also vary depending on how much of the fuel's surface is exposed to the air. As the exposed surface of a fuel increases in comparison to its volume, the burning rate of the fuel increases as well. When the fuel's gases have more surface area from which to escape, they can come into contact with more air. The increased exposure to air increases the amount of oxygen available for combustion. For example, people often use small twigs and pieces of wood called kindling to start a campfire. Kindling has a large amount of surface area compared to its volume. Its relatively large surface area to volume ratio also means that kindling heats and ignites more easily than thicker pieces of wood. Once ignited, kindling burns very quickly. C Products of Combustion The products that a fire releases, and the rate at which it releases them, depend on the fuel and on the fire's burning rate. Some fuels will produce more heat than others as they burn, and some will produce different kinds of gases. A fire that burns slowly may produce different products than one that burns quickly. The burning rate also affects the rate at which a fire releases products. C1 Light and Heat Once a material ignites, a flame forms. The flame consists of volatile gases moving upward, and it is the region in which the combustion reaction occurs. The gases in the flame move upward because they are hotter--and therefore lighter--than the surrounding air. The colors in the flame come from unburned carbon particles that glow and travel upward as the flame heats them. The flame continues to burn as the volatile gases streaming from the fuel combine with oxygen from the surrounding air. Different parts of the flame have different temperatures. Most common fuels are compounds called hydrocarbons, and they produce about the same flame temperature, roughly 1200°C (2200°F). The maximum theoretical flame temperature for most hydrocarbons is about 1300°C (2400°F). Different fuels produce varying amounts of heat. The rate at which a fire generates heat is equal to the burning rate of the fuel (measured in grams per second, or g/s) multiplied by the amount of heat produced by the combustion reaction. This second factor is called the effective heat of combustion, and scientists measure it in units of kilojoules per gram (kJ/g). When a gram of wood burns, for example, it produces 8 kJ of heat energy. Wood's effective heat of combustion is therefore 8 kJ/g. Polyurethane's effective heat of combustion is about 18 kJ/g. Polyurethane's burning rate is also about twice that of wood under similar conditions. Multiplying the burning rates for these two substances by their effective heats of combustion, one finds that polyurethane fires produce heat at about 4.5 times the rate of wood fires under similar conditions. C2 Gases Fires can produce a number of different gases, including some that are harmless and some that are toxic. Carbon dioxide (CO2) and water vapor (H2O) are two relatively harmless gases produced by fires. Toxic gases from fires include carbon monoxide (CO), hydrogen cyanide (HCN), sulfur dioxide (SO2), and hydrogen chloride (HCl). The specific gases and the amount of gas a fire produces depend on the type of fuel involved and the environment surrounding the fire. Different fuels will react differently in the combustion reaction, producing gases and amounts of gas specific to that type of fuel. For example, in well-ventilated conditions, polyurethane foam produces ten times more carbon monoxide for each gram burned than does wood. Fires that burn in an oxygen-rich environment will also produce less carbon monoxide than fires that burn where little oxygen is present. A well-ventilated fire has plenty of oxygen, so nearly all of the fuel's volatile gases can take part in the combustion reaction, combining with oxygen in the air to produce carbon dioxide and water vapor. These fires produce less carbon monoxide because there is less carbon and oxygen left over from the initial combustion reaction to form carbon monoxide. Fires that occur in an environment lacking sufficient oxygen will burn incompletely and smolder. These fires produce increasing amounts of carbon monoxide. For example, in an enclosed room, a fire will use up oxygen from the air as it progresses, decreasing the amount of oxygen in the room over time. Without sufficient oxygen, the volatile gases from the fire cannot fully take part in the combustion reaction. Some of the gases instead react to form carbon monoxide, which requires less oxygen than does combustion. Eventually, the amount of oxygen decreases below the level necessary for continued combustion, causing the fire to self-extinguish. Depending on the type of fuel, most fires self-extinguish at an oxygen concentration between 12 and 14 percent (by volume). By contrast, normal atmospheric air has an oxygen concentration of 21 percent. C3 Soot As fires produce light, heat, and gases, they also produce soot, consisting of mostly carbon particles. Smoke may be defined either as just the soot particles given off by a fire, or as both the soot and the gaseous products of combustion. The amount of soot produced by a fire depends on the type of fuel, the fuel's burning rate, and environmental conditions. Most plastic fuels produce more soot than wood and other cellulose fuels. Plastics also usually burn more quickly than wood. Under similar conditions, for example, a slab of polyurethane will burn almost twice as fast as a slab of wood. The composition of plastic and plastic's more rapid burning rate cause it to produce about 2.7 times as much soot as does wood. Fires also tend to produce more soot when they smolder and less soot when they burn freely in a well-ventilated area, with plenty of oxygen available. VI DESTRUCTIVE FORCE OF FIRE Destructive fires can occur wherever fuel and oxygen are available, including in office buildings, homes, vehicles, and forests. According to the National Fire Protection Association, a fire broke out in a building or structure every 61 seconds in the United States in 1998. Three-quarters of all structure fires in the United States and Canada occur in people's homes. In 1998 there were about 1,756,000 fires in the United States. These fires led to 4,000 deaths, 23,000 injuries, and $9 billion in property damage. Every 76 seconds a motor vehicle fire occurred, for a total of 413,500 such fires. In 1995, the most recent date for which Canadian statistics are available, Canada had about 64,000 fires. These fires resulted in 400 deaths, 3,600 injuries, and $1 billion (Canadian) in direct property damage. Extinguishing a fire involves removing one of the requirements for combustion. Firefighters may physically remove the fuel from the fire by taking a burning item outside a structure. They can remove heat by cooling the fire with water or remove oxygen by smothering the fire with chemicals or a fire blanket. Interrupting the chemical chain reaction is more difficult but is typically done by applying special chemicals, such as halogenated compounds, to the fire. These halogenated compounds are being used less often as they cause damage to the atmosphere's ozone layer. A House Fires Many people worry about being trapped in a hotel fire or a fire at their school or workplace. Yet about 80 percent of all U.S. and Canadian fire fatalities are caused by fires in the home. In 1998 house fires accounted for 381,500, or 21 percent, of all fires that occurred in the United States. In Canada 25,747 house fires occurred in 1995--about 40 percent of all Canadian fires that year. Most house fires result from cooking accidents in the kitchen. Cigarettes, however, cause the majority of house fires that turn deadly. Smoking-related fires tend to smolder before they are discovered. Once a fire breaks out, it can envelop a room within minutes. Temperatures in the room may exceed 600°C (1100°F). While this heat alone would be deadly, the toxic gas in the smoke causes the majority of deaths and injuries. Almost half of all fatalities from fires are due to carbon monoxide poisoning, and more than a third are due to cardiopulmonary complications. People protect themselves from the dangers of house fires in several ways. Fire extinguishers in homes enable people to put out fires before they become dangerous, while smoke detectors alert residents that a fire has broken out in the fire's early stages. Communities support either a local fire department or a volunteer force, so people can call a phone number to summon firefighters to their home. The furniture, clothing, and building industries help prevent fires in the home by making products out of nonflammable materials or by treating materials with chemicals to make them less flammable. B Fires in the Workplace Dangerous work conditions and arson can lead to fires in the workplace. Industries that produce chemicals often deal with extremely flammable materials, while metalworking industries deal with materials at very high temperatures. Companies prevent fires by training employees in the handling of dangerous materials and by hiring specialists, called fire protection engineers, to design safe workspaces. Sprinkler systems can limit property damage, and the establishment of clear exit routes for employees can limit injury caused by fire. In the United States, the leading cause of fires in office buildings is arson. Office buildings often include security systems, such as locked doors and camera surveillance of entrances and exits, to prevent potentially dangerous people from entering the building. C Wildland Fires Wildland fires occur in undeveloped areas of land and are fueled by forest or grassland vegetation. The leading causes of wildland fires are lightning and human-caused ignitions, including those from equipment exhaust, abandoned campfires, cigarettes, and arson. Such fires destroy forested areas as well as homes and property bordering these areas. Small-scale, periodic wildland fires can actually improve the health, resilience, and productivity of an ecosystem. When these fires do not occur often enough, however, flammable vegetation can build up, leading to a large-scale fire that harms plant and animal species. Education and regular patrols of campgrounds help prevent or control many of the fires caused by people. Intentionally setting controlled, small-scale fires prevents fuel from building up. To battle dangerous wildland fires, firefighters use airplanes to apply chemicals from the air and trucks and pumps to apply water at the ground level. They also remove vegetation surrounding the fire to create a border that lacks fuel. The destruction caused by a wildland fire, and the effectiveness of firefighters in extinguishing it, depend on the terrain, the type of vegetation in the area, weather conditions, and the availability of firefighting resources. Contributed By: Jonathan R. Barnett Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.