The combustibility is the degree of reactivity of a compound to vigorously exothermic reacting with oxygen or another oxidizing (oxidizing) agent. It not only applies to chemical substances, but also to a wide range of materials, which are classified by Building Codes based on it..
Therefore, combustibility is extremely important in establishing how easily matter burns. From here, flammable substances or compounds, fuels, and non-fuels are released..
The combustibility of the material depends not only on its chemical properties (molecular structure or stability of the bonds) but also on its surface-volume ratio; that is, the greater the surface area of an object (such as slush dust), the greater its tendency to burn..
Visually, its incandescent and flaming effects can be impressive. The flames with their shades of yellow and red (blue and other colors), are indicative of a latent transformation; although it was formerly believed that atoms of matter were destroyed in the process.
The studies of fire, as well as that of combustibility, involve a dense theory of molecular dynamics. In addition, the concept of autocatalysis, because the heat from the flame “feeds” the reaction so that it does not stop until all the fuel has reacted
For that reason perhaps fire sometimes gives the impression of being alive. However, in a strict rational sense, fire is nothing more than energy manifested in light and heat (even with the immense molecular complexity of the background).
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Known in English as Flash point, is the minimum temperature at which a substance ignites to start combustion.
The entire fire process begins with a small spark, which provides the necessary heat to overcome the energy barrier that prevents the reaction from being spontaneous. Otherwise, the minimum contact of oxygen with a material would cause it to burn even under freezing temperatures..
Flash point is the parameter to define how combustible a substance or material can or cannot be. Therefore, a highly combustible or flammable substance has a low flash point; that is, it requires temperatures between 38 and 93ºC to burn and unleash a fire.
The difference between a flammable and combustible substance is governed by international law. This being the case, the temperature ranges considered may vary in value. Also, the words 'combustibility' and 'flammability' are interchangeable; but they are not 'flammable' or 'combustible'.
A flammable substance has a lower flash point compared to a combustible substance. For this reason, flammable substances are potentially more dangerous than fuels, and their use is strictly supervised..
Both processes or chemical reactions consist of a transfer of electrons in which oxygen may or may not participate. Oxygen gas is a powerful oxidizing agent, whose electronegativity makes its O = O double bond reactive, which, after accepting electrons and forming new bonds, releases energy.
Thus, in an oxidation reaction Otwo gains electrons from any sufficiently reducing substance (electron donor). For example, many metals in contact with air and moisture end up rusting. Silver darkens, iron reddens, and copper can even turn patina..
However, they do not give off flames when doing so. If so, all metals would have a dangerous combustibility and buildings would burn in the heat of the sun. This is where the difference between combustion and oxidation lies: the amount of energy released.
In combustion an oxidation occurs where the heat released is self-sustaining, bright and hot. Likewise, combustion is a much more accelerated process, since any energy barrier between the material and oxygen (or any oxidizing substance, such as permanganates) is overcome..
Other gases, such as Cltwo and the Ftwo they can initiate vigorously exothermic combustion reactions. And among the oxidizing liquids or solids are hydrogen peroxide, HtwoORtwo, and ammonium nitrate, NH4NOT3.
As just explained, it must not have too low a flash point, and it must be able to react with oxygen or oxidizer. Many substances enter this type of material, especially vegetables, plastics, wood, metals, fats, hydrocarbons, etc..
Some are solid, others liquid or fizzy. Gases are generally so reactive that they are considered, by definition, as flammable substances.
Gases are the ones that burn much more easily, such as hydrogen and acetylene, CtwoH4. This is because the gas mixes much faster with the oxygen, which equates to a larger contact area. You can easily imagine a sea of gaseous molecules colliding with each other right at the point of ignition or ignition..
The reaction of gaseous fuels is so fast and effective that explosions are generated. For this reason, gas leaks represent a high risk situation..
However, not all gases are flammable or combustible. For example, noble gases, such as argon, do not react with oxygen.
The same situation occurs with nitrogen, due to its strong N≡N triple bond; however, it can rupture under extreme pressure and temperature conditions, such as those found in a thunderstorm..
How is the combustibility of solids? Any material subjected to high temperatures can catch fire; however, the speed with which it does so depends on the surface-to-volume ratio (and other factors, such as the use of protective films).
Physically, a solid solid takes longer to burn and spreads less fire because its molecules come in less contact with oxygen than a laminar or pulverized solid. For example, a paper row burns much faster than a block of wood of the same dimensions..
Likewise, a pile of iron powder burns more vigorously compared to an iron sheet..
Chemically, the combustibility of a solid depends on which atoms compose it, their arrangement (amorphous, crystalline) and the molecular structure. If it is composed mainly of carbon atoms, even with a complex structure, the following reaction will occur when burning:
C + Otwo => COtwo
But the carbons are not alone, but accompanied by hydrogens and other atoms, which also react with oxygen. Thus, HtwoBEAR3, NOTtwo, and other compounds.
However, the molecules produced in combustion depend on the amount of reacting oxygen. If carbon, for example, reacts with an oxygen deficit, the product is:
C + 1 / 2Otwo => CO
Note that the COtwo and CO, COtwo it is more oxygenated, because it has more oxygen atoms. Therefore, incomplete combustions generate compounds with fewer O atoms, compared to those obtained in complete combustion..
In addition to carbon, there may be metallic solids that withstand even higher temperatures before burning and giving rise to their corresponding oxides. Unlike organic compounds, metals do not release gases (unless they have impurities), since their atoms are confined to the metallic structure. They burn where they are.
The combustibility of liquids depends on their chemical nature, as is their degree of oxidation. Very oxidized liquids, without many electrons to donate, such as water or tetrafluorocarbon, CF4, do not burn significantly.
But, even more important than this chemical characteristic, is its vapor pressure. A volatile liquid has a high vapor pressure, which makes it flammable and dangerous. Why? Because the gaseous molecules "prowling" the surface of the liquid are the first to burn, and represent the focus of the fire.
Volatile liquids are distinguished by strong odors and their gases quickly occupy a large volume. Gasoline is a clear example of a highly flammable liquid. And regarding fuels, diesel and other heavier hydrocarbon mixtures are among the most common..
Some liquids, such as water, cannot burn because their gaseous molecules cannot give up their electrons to oxygen. In fact, it is used instinctively to put out flames and it is one of the substances most applied by firefighters. The intense heat of the fire is transferred to the water, which uses it to change to the gaseous phase.
They have been seen in real and fictitious scenes how the fire burns on the surface of the sea; however, the true fuel is oil or any oil immiscible with water and floating on the surface.
All fuels that have a percentage of water (or moisture) in their composition, have as a consequence a decrease in their combustibility.
This is due, again, to the fact that some of the initial heat is lost by heating the water particles. For this reason, wet solids do not burn until their water content is eliminated..
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