COMBUSTIBILITY: FLASH POINT AND CHARACTERISTICS - CHEMISTRY - 2025

The combustibility is the degree of reactivity of a compound to react vigorously exothermic manner with oxygen or other oxidizing agent (oxidizer). 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 the ease with which matter burns. From here, flammable substances or compounds, fuels, and non-fuels are released.

Source: Pxhere

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 participates, because the heat of 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 in the background).

Flash or ignition point

Known in English as Flash Point, it is the minimum temperature at which a substance ignites to start combustion.

The entire process of fire begins with a small spark, which provides the necessary heat to overcome the energy barrier that prevents the reaction from being spontaneous. Otherwise, the minimal contact of oxygen with a material would cause it to burn even under freezing temperatures.

The 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 that of a combustible substance. For this reason flammable substances are potentially more dangerous than fuels, and their use is strictly supervised.

Differences between combustion and oxidation

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, O ₂ 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 a patina color.

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 Cl ₂ and F ₂ can initiate vigorously exothermic combustion reactions. And among the oxidizing liquids or solids are hydrogen peroxide, H ₂ O ₂, and ammonium nitrate, NH ₄ NO ₃.

Characteristics of a fuel

As just explained, it should not have too low a flash point, and should 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, in general, are so reactive that they are considered, according to the definition, as flammable substances.

-Gas

Gases are the ones that burn much more easily, such as hydrogen and acetylene, C ₂ H ₄. This is because the gas mixes much faster with the oxygen, which is equal to a larger contact area. You can easily imagine a sea of ​​gaseous molecules colliding with each other just 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 triple bond N≡N; however, it can rupture under extreme pressure and temperature conditions, such as those found in an electrical storm.

-Solid

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.

Also, a pile of iron powder burns more vigorously than a sheet of iron.

Organic and metallic compounds

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 + O ₂ => CO ₂

But the carbons are not alone, but accompanied by hydrogens and other atoms, which also react with oxygen. Thus, H ₂ O, SO ₃, NO ₂, and other compounds are produced.

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 / 2O ₂ => CO

Note that between CO ₂ and CO, CO ₂ is more oxygenated, because it has more oxygen atoms. Therefore, incomplete combustions generate compounds with a lower number of 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.

Liquids

The combustibility of liquids depends on their chemical nature, as is their degree of oxidation. Highly oxidized liquids, without many electrons to donate, such as water or tetrafluorocarbon, CF ₄, 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 when it comes to fuels, diesel and other heavier hydrocarbon mixtures are among the most common.

Water

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 most applied substances by firefighters. The intense heat from the fire is transferred to the water, which uses it to change to the gaseous phase.

They have been seen in real and fictional scenes how the fire burns on the surface of the sea; however, the real 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 again because 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 removed.

References

1. Chemicool Dictionary. (2017). Definition of Fuel. Recovered from: chemicool.com
2. Summers, Vincent. (April 5, 2018). Is Nitrogen Fuel? Sciencing. Recovered from: sciencing.com
3. Helmenstine, Anne Marie, Ph.D. (June 22, 2018). Combustion Definition (Chemistry). Recovered from: thoughtco.com
4. Wikipedia. (2018). Combustibility and flammability. Recovered from: en.wikipedia.org
5. Marpic Web Design. (2015, June 16). What types of fires are there and how is the combustibility of the materials that define this typology? Recovered from: marpicsl.com
6. Learn Emergencies. (sf). Theory of fire. Recovered from: aprendemergencias.es
7. Quimicas.net (2018). Examples of Flammable Substances. Recovered from: quimicas.net