Is Air Flammable? (A comprehensive overview)

Is Air Flammable?

No, air is not flammable. The vast majority of the gases that compose the common are non-flammable. The one flammable gas in the air is methane, but its presence in the air is far under its flammability range.

However, the air can and does take part in fires and in explosions. More specifically, the oxygen (O2) in the air plays an important role in the chemical reaction that generates fires or explosions. This chemical reaction is referred to as combustion reaction.

When is Something Considered Flammable?

A flammable substance is one that can be oxidized during a combustion reaction that produces high enough energy for fire to be generated. And  air, in ordinary conditions, does not contain any substance that can be oxidized in such a manner.

Some examples of substances that can be oxidized at a fast enough rate to generate a fire include: methane, ethanol, metallic sodium. The ease with which these substances are oxidized indicates how flammable they are.

What is the Role of Air in a Combustion Reaction?

A combustion reaction occurs when a substance is oxidized in an highly exergonic reaction (that is, giving off a high amount of heat). Not every combustion reaction will result in fire. For that the substances must go to the gas phase.

The substance that is oxidized can be called the fuel. The substance that oxidizes the fuel is called the oxidizer. In the case of a combustion reaction that occurs in the presence of air, O2 is usually the oxidizer. In these chemical reactions, it is necessary to have a source of heat in order for the reaction to start. 

Once a combustion reaction starts, the reaction itself produces more heat. A portion of this heat (thermal energy) can then act in the continuation of the combustion reaction. The fuel is the component that can have flammability. 

The easier the combustion reaction can start, the higher the flammability of the fuel. Even though oxygen takes part in the reaction, by definition it cannot be flammable because the role oxygen plays is that of an oxidizer not a fuel.

Some Facts About Fire

Fire is the result of some of the energy released during a combustion reaction. As the reagents (e.g. methane and O2) transform into products (e. g. H2O and CO2) a relatively large amount of energy is released. 

This energy comes from the chemical bonds that are formed during the progress of the chemical reaction. An amount of this energy is absorbed by the electrons of the molecules present in this system in the gaseous phase, then the electrons immediately release this energy as photons.

As a result, we can see the flames. The color of the flame can be an indicator of the temperature. In a standard candle, the blue portion of the flame has a temperature of 1000-1400 °C, whilst the yellow portion has temperatures of 800-1000 °C.

What is Air Composed of?

Air is how the atmosphere of the Earth is commonly known. Ambient air is the ambient air in a particular location3 and so ambient air can have a very different composition from place to place. 

In dry air (that is without any water ) the amount of the  components is relatively  constant (percent by volume given): N2 (78.1%), O2 (20.9%), Ar (0.9%), and also CO2, Ne, He, methane; Kr, H2, nitrous oxide, Xe (composing the remaining portion). 

The concentration of CO2, methane, nitrous  oxide, and  some  other  gases produced by human activities are increasing with time. Since none of the gases in the air can act as a fuel (in high enough concentrations methane can act as fuel), air has no flammability.

Difference Between Compressed Air and Canned Air 

Compressed air is air under high pressure. So compressed air, the same way as the regular air, is not flammable. It is worth mentioning that the commonly called canned air, used to clean electronics, is not the same as air from the Atmosphere that was compressed. 

Canned air is composed of a mixture of highly flammable substances. Therefore they are explosive in the presence of fire or heat.

Why Does Fire Need Oxygen?

As mentioned above, combustion reactions require an oxidizer. The oxygen present in the air (O2) is a strong oxidizer. A fire also requires a fuel, the fuel can be from many sources including wood and leaves or it can also be petroleum derivatives such as gas.

For a combustion reaction to start, energy is necessary. That energy can be a sparkle or it can be simple mechanical processes such as friction. That energy can also come from another chemical reaction that releases enough heat.

Once a fire starts, as long as there is fuel to burn and oxygen accessible in a sufficient concentration (at least 10%, at 1 atm), the fire will continue. If either the oxygen or the fuel are properly cut out,  the fire will extinguish.

Is Oxygen Stable or Unstable?

One may wonder why oxygen is capable of promoting such explosive reactions, given that it is present in the air around us and within living beings. 

Part of the answer lies in the difference of potential energy between oxygen (O2) and the molecule in which it transforms (CO2 in the case of fossil fuels). Carbon dioxide is more stable than oxygen, in other words, it has less potential energy. 

In many combustion reactions water is also formed, water is also quite stable. 

In nature molecules will go to a lower energy state when possible. This difference of potential energy between the reactants (fuel and oxygen) and the products (carbon dioxide and water).

In loose terms, the flammability could be inferred from how unstable the fuel is. The more unstable a molecule is, the more promptly  it tends to react. Therefore less energy (heat) is necessary for this fuel to start the combustion reaction. 

The less heat it is necessary, the higher is the flammability of a given fuel.

Are Other Gases Flammable?

A gas is considered flammable when having a flammable range with air at 20 °C and a standard pressure of 101.3 kPa (1 atm). Examples of flammable gases include methane and butane, which are commonly used by the global population. 

A flammable gas that is capable of reacting explosively even in the absence of air or oxygen is called a chemically unstable gas. Examples include acetylene and ethylene, which are highly used in the polymer industry.

Does Air Contain Flammable Gases?

Air contains methane, which is in fact a flammable substance. In the standard pressure (1 atm) methane becomes flammable at a concentration of around 5.4% (methane concentration in the atmosphere is around 0.00017%). 

So air does have flammable gases (methane) in it. Still, that gas is not present in a flammable concentration. What determines if the air in those cases will be flammable is the concentration of the flammable substances in it.

Can a Liquid be Flammable?

A liquid that has a flash point lower or equal to 93 °C is considered flammable. In order to determine how flammable a liquid is, data on its flash point and initial boiling point are needed. Data for a wide range of liquids can be found in the literature. 

Examples of flammable liquids include: Methanol, Ethanol and Gasoline (mostly octane).

What is Flash Point?

The flash point of a flammable liquid is the lowest temperature at which the vapors (molecules that have transitioned from the liquid phase to the gas phase) of said liquid will ignite if given an ignition source. 

Flash points for some liquids at ambient pressure.

LiquidFlash Point (°C)
methanol 11 to 12
ethanol13
octane 13
ammonia132

Only the vapor is considered, since the liquid itself will usually ignite regardless of the temperature. The lower the flash point of a liquid the more flammable it is considered, and hence the higher its flammability.

Conclusion

From this text about air and flammability it is possible to conclude that it is important to distinguish between the words flammability, oxidizer and fuel. As these terms are related to each other and all have significant importance, but they have very distinct definitions.

 Although a highly reactive oxidizer such as oxygen is present in approx. 20% of the air, that is not enough for it to be considered dangerous under most circumstances. (Please feel free to comment on the content or ask any questions in the comments section below).

Frequently asked questions (FAQ): Is Air Flammable?

Is Oxygen actually flammable?

Oxygen itself is not flammable per se, but it can cause other substances to ignite more easily and to burn way faster. Given this high reactivity of oxygen, it can cause fires or explosions but not without a substance that can act as fuel. The easier the fuel reacts, the higher its flammability.

Can pure oxygen explode?

Yes it can. Unlike air, nitrogen, helium and other inert gases, oxygen in high concentrations (such as an oxygen cylinder) is extremely reactive. In the presence of oil, petroleum derivatives and paper for example, oxygen can cause explosions.

Can fire be made from air?

Yes, fire could be made from air, in some specific cases. If the air in questions contains something that can be used as fuel, for example, sawdust or methane.

Can hot air start a fire?

Under specific conditions, hot air or any air could start a fire. For instance if a ignition source and a fuel are in contact but no oxidizer (such as air for example) is present there should be no fire. But in the moment that air goes in contact with the fuel and the ignition source a fire would start.

Is burning possible without oxygen?

Yes it is possible for a fire to start and continue without oxygen. What is necessary for the fire to burn is an oxidizer, molecular oxygen (O2) just happens to be the most accessible oxidizer on Earth. For example, the halogen gases fluorine (F2) and chlorine (Cl2) can react explosively with methane gas.

Does the sun have oxygen?

The Sun does not have oxygen in any appreciable amount, like the rest of the universe, the Sun is made mostly of hydrogen. There isn’t enough oxygen in the entire solar system to keep the surface of the sun burning through chemical combustion for more than a very short time. Instead, the sun’s heat and light comes from thermonuclear fusion of hydrogen into helium which releases far more energy than a combustion reaction.

References

Definition of pure air according to IUPAC Page 2172 of Calvert, J. G.. “Glossary of atmospheric chemistry terms (Recommendations 1990)” Pure and Applied Chemistry, vol. 62, no. 11, 1990, p. 2167-2219. https://doi.org/10.1351/pac199062112167

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook.

https://unece.org/fileadmin/DAM/trans/danger/publi/ghs/ghs_rev05/English/02e_part2.pdf

Methods to more specifically determine how flammable something is can be found at: ISO – ISO 10156:2017

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