Fire is the visible sign of a chemical reaction, the substance that burns is combined with the oxygen of the air. Only gases can burn with Flame. Therefore, in order to obtain a flame from a solid or liquid fuel, you should gasify it.
A substance begins to burn when it warms to a certain temperature, the
called TEMPERATURE OF IGNITION. During combustion it is produced heat that
maintains a temperature above the ignition, so that the reaction can continue.
If the temperature becomes quite high so that the substance begins to gasified,
the luminous phenomenon we have been talking about appears, our friend
the fire.
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All combustion in the air (with or without Flame), is the combination of a substance and the oxygen. If we see it from the chemical point of view, we could say that the combustion is a quick chemical reaction between two substances that it is generally accompanied for the generation of heat and light in form of Flame. In most of the cases, the oxygen (O2) is part of the reagents. The chemical reaction this way produced is an oxidation, and the new substances that are formed are called oxides. |
At this time, it would be good to mention that, besides the chemical processes in the combustion, we meet with physical type processes. In the gassy type combustion, for example, the diffusion of reagents and the products of the combustion, depends in their concentration, pressure and changes of temperature, as well as the diffusion of coefficients.
The combustion can also emit luminous energy, especially in the infrared portion of the spectrum. The light emitted by a Flame, is originated by the presence of particles in state of electric excitement and by ions, radicals and electrons.
Another detail we find among the curiosities of the combustion, is that all the average fuels contain Carbon (C). The coal and the coke are more or less carbon pure; the gas of the city, the butane, the gasoline, the petroleum and the heavy oil are organic substances of carbon and hydrogen, better known as Hydrocarbons.
In a complete combustion (in the one that all the combustible components are consumed), starting from the carbon, carbon dioxide is formed (CO2); starting from hydrocarbons, we obtain carbon dioxide and water. If the quantity of oxygen is insufficient for a complete combustion, carbon monoxide is obtained (CO) which is a poisonous gas. The fire of a chimney whose ventilation duct is very closed, so that little oxygen enters, can cause dangerous intoxication by carbon monoxide. If the combustion of coal and wood is very incomplete, pitch is formed. Most of the fuels contain mineral substances that aren’t burned and they remain in form of ash.
The color of a pure gassy Flame, depends on the substance that burns.
The flame of hydrocarbons is blue if a precise amount of oxygen is used.
The color is almost
independent of the temperature of the Flame. If the air is substituted
by pure oxygen (remember that the air is a mixture of gases, mainly Nitrogen
and Oxygen), for
example, in a torch, the Flame warms more and it shines with more intensity,
but it continues being of blue color. When a solid body is warmed, it adopts
different
colors according to the temperature. The incandescent solid particles
of the fuel are those that give the Flame their colors, from the dark red
in the 1112 - 1472 °F, the orange and the yellow in the 2012 - 2192
°F, until reaching the blinding white color above the 2732 °F.
Let’s see an example of the coloration of the Flame. When an estuarine
candle burns, it melts a part of itself, it is gasified and a Flame is
formed. The temperature of
the interior of the Flame is inferior to the exterior one, because
it is not in direct contact with the air. Incandescent particles of estuarine
give the color to the Flame. If a clod of sugar is added, particles of
carbon of this Flame are adhered to its surface.
The luminous colors of fire come from a series of particles of carbon
that float inside fire. All the bodies that burn produce light whose color
depends on the
temperature that take place. High temperature gives light, preferably
of the blue area of the spectrum; low temperature emits a color of the
light that goes from yellow and orange to red.
You may have wondered why wood is a good fuel?
This is due to that the wood is made up from a series of substances,
mainly of cellulose and lignin, both composed fundamentally by carbon,
hydrogen and oxygen. When a substance is warmed up, it receives an energy
increase and the movements of the molecules become more intense. These
molecules then, are able to break up with more easiness and the atoms to
combine with other, of oxygen, forming new substances, this is, combustion
takes place.
The big cellulose molecules fragment in smaller products (carbon dioxide), of lower chemical energy. The energy difference is presented as electromagnetic radiation in form of heat and light. Some combustible particles that float in the fire aren't burned, and they become soot. The mineral elements of the wood subsist as ash.
Combustion without Flame?
Of course it exists!, many times we don't realize that the reality
of our daily world is, sometimes, much more impressive that the one created
in science fiction. Do you know that yourself is a clear example of a combustion
without Flame? At once I will explain why... In the nature many processes
take place that from the chemical point of view, are combustion without
Flame. In any case, a combination between a substance and oxygen is developed,
where heat is produced. Combustion of this type are the oxidation of iron
(Fe), the transformation of food in corporal energy and rot.
If a Flame is not formed it is because the process is carried out so slowly that the combustion doesn't reach the ignition temperature. The complete combustion, for example, of 1 kg. of sugar producing carbon dioxide and water, emits the same quantity of heat, either it is carried out outdoors or carried out in a closed system, as the human body is. The man's temperature stays to about 98 °F because the heat of the reaction, relatively quick, is absorbed by the man's corporal mass. The energy provided by the combustion that is developed in our digestive tube is used, above all, to maintain the temperature and the muscular energy, as well as for the construction of new cells.
The rust of the metals consists on a corrosion of the surface, because of the oxidation. Many metals are oxidized in contact with the air, being covered by a layer of metallic oxide. In certain cases, this cover is very hard and thick and it protects the metal against the later attack of the oxygen of the air. An excellent example of this would be the aluminum, where the layer is so thick that the metal shines and it is not attacked. On the contrary, the rust in the case of the iron, is a porous layer that is formed when the oxygen of the air crosses and corrodes the iron.
Another combustion without Flame, is the rot. For example, the rot of the manure is comparable to the one of human body. If the heat is not eliminated fast enough, the temperature of the manure can rise in such a way that may cause its fire. Remember that the combustion begins when the temperature of ignition of a substance is reached that will act as fuel. Another example of a spontaneous fire, would be when we put in contact a cloth soaked in gasoline with a heap of wet hay.
Many substances that burn with difficulty can burn if the oxygen and the heat reach to all their parts. Even, iron is able to burn when it comes undone in fine chips, since these offer to the oxygen a great contact surface. This principle is easy to check, is more difficult to make a wound paper burn that one wrinkled. Likewise, a firewood trunk burns more easily if some cuts are made.
This is one of my favorite topics... The explosive combustion. The explosive combustion is a very quick oxidation that liberates energy in form of heat. In the reaction gases are formed that are expanded by being warmed up producing a strong pressure. The modern technique takes advantage of this effect in several ways: to impel automobiles and airplanes (we have the example of the internal combustion motor), and as explosive for the mines and construction of tunnels. However also it has been used in a destructive way, as the creation of weapons and bombs.
The oldest explosive is the black gunpowder, the only one existent until half-filled the XIX century, in which the use of the nitroglycerine began. In 1867, Nobel was able to transform the nitroglycerine in another less dangerous explosive, the dynamite. During the last hundred years, it has been possible to manufacture a series of more potent and safer explosives.
Nowadays, much advantage from the explosive combustion has been taken to devise revolutionary inventions in the field of communications. In the explosion motor, the pressure of the gas that is formed in the explosion becomes motive force. This phenomenon has allowed the development of the motoring and of the aviation increase.
The explosive combustion is the most intense of all, but its explosion force cannot be compared with the one that provides the atomic energy. In the combustion, an energy transformation takes place in which the atoms enter in new combinations. In the atomic explosion, an explosion of the atomic nucleus takes place, process different from combustion… well, that is a topic of which we won't talk about in this page. As measure of the effect of an atomic explosion the comparison with the trinitrotoluene (TNT) is used, one of the most potent explosives. An 20 megatons atomic bomb has an equivalent explosive force to 20 millions of tons of TNT.
Explosion.
Everybody should be very familiarized with this term. An explosion
is produced when an explosive substance, for example nitroglycerine, is
exposed to heating, or takes a hit, and becomes gas and a high pressure
takes place. The imbalance taken place by the expansion unchains an explosion
and a strong wave of pressure. The process is very quick and the explosion
may destroy or throw near objects far away.
With this concludes the part that corresponds to Combustion. I hope that you have learned much of this because in this way you will be able to understand better what the fire is, and not to see it like a threat but as an ally that will make your life much easier.