You may have seen signs that indicate that you shouldn’t smoke because oxygen is present. This surely must mean that oxygen is flammable, right?
It’s not quite that simple. If oxygen (O2) was flammable, then the ambient air that we breathe, which has oxygen in it, would catch fire each time you lit a match.
Since that doesn't happen, there must be another explanation.
In this article, find out whether oxygen and anything that contains the element is really flammable.
Is Oxygen Flammable?
The good news is that oxygen alone is not flammable. However, it does aid the combustion process when it interacts with other elements.
This can cause fires, and it also helps a fire to burn faster and hotter. While oxygen does not act as a fuel for the flames, it does help it to increase the fire.
While oxygen’s role as an oxidizing agent in a fire seems very minor, it is an essential part of the fire triangle.
How Oxygen Behaves As Part Of Fire
Chemists, scientists and experienced firefighters are all well aware of the essential role that oxygen plays in a fire.
Oxygen aids the combustion reaction but this does not mean that it is flammable. If you took pure oxygen and tried to ignite it, then nothing would happen.
Signs such as “Danger, Oxygen Present, Highly Flammable” are precautions to ensure that, if there is a fire breaking out, the presence of oxygen may make the fire bigger and burn hotter.
There is a famous experiment to prove how oxygen works in a fire. Simply take a mouthful of pure oxygen, then blow it onto a lit cigarette.
Typically, a cigarette burns with a red and orange end. When you blow the oxygen, you will be able to see that the end turns yellow, almost white.
This means that it is much hotter and burning the cigarette much faster. This is exactly how oxygen behaves in all fires.
Oxygen doesn’t burn the cigarette, instead it aids the combustion process making the cigarette burn faster and hotter.
It acts as a kind of fertilizer for the flames.
The Fire Triangle
We already briefly mentioned the fire triangle, and how oxygen plays a significant part in it.
The fire triangle simply describes the three elements that need to be present in order for a fire to happen.
These three elements are fuel, heat and an oxidizer (such as oxygen). These elements will allow a fire to form and they will also contribute to how long and hot the fire can burn.
The fire triangle and the knowledge of how each element in the triangle behaves is essential (and life saving) for firefighters.
Essentially, firefighters always try to remove at least one of these three elements. They can cool the fire with water or another extinguishing agent.
Firefighters can also remove any of the fire fuel by removing material that feeds the fire. For example, in a wood fire they would dig a path or trench to cut off the fire from another section of the forest.
In order to extinguish a fire you can also simply remove the oxygen. For smaller kitchen fires this can be as simply as placing a lid over a burning pan or throwing a fire blanket over a fire source.
However, removing oxygen from the fire isn’t always the quickest or most effective method. Strong fires with a high temperature residual heat are persistent.
You will need to keep the blanket or lid on top of the fire for some time, until the fire burns out.
If you remove the cover too soon, the residual heat would be strong enough to use the oxygen from the air to reignite the fire again.
Whenever there is oxygen presented in a very hot area, the oxygen can support the fire. However, the oxygen does not fuel the flames.
There are a number of different people who use the fire triangle for their jobs handling fires and fire risks.
In recent years, scientist and firefighters also added a fourth dimension to the triangle, which now makes it a tetrahedron.
This fourth element in the triangle is called free radicals. Free radicals are the sum of all the different flammable elements which can also exacerbate a fire.
It is important to understand the chain reaction in a fire from a chemical point of view.
The elements that fall into the free radicals category can be easily eliminated with a dry powder or extinguishing agent.
This can make a fire much smaller and less dangerous.
They can also be eliminated by a layer of dry chemical agents. The removal of free radicals can easily be accomplished in smaller fires.
However, it is not just the different elements that are involved in a fire that can make extinguishing a fire a very difficult task.
More complex fires, such as structures fires, that involve structural components of a building are also more difficult to put out.
These usually include large buildings, such as commercial and industrial buildings, as well as barns.
Fires within these buildings produce a large amount of smoke and other floating elements. This means that firefighters need to use much more water and other ways to extinguish the fire.
In addition, structural fires also produce more oxygen gases which contribute to an increase in temperature in the fire.
Together with the respiratory challenges that firefighters face with the additional gases, this makes it much more difficult to extinguish the fire.
Gases emitted from the combustion of fires can be dangerous even when the fire is just smoldering.
This poses the risk of breathing in these toxic gases and firefighters have to wear breathing apparatus to protect themselves.
Is Liquid Oxygen Flammable?
Liquid oxygen is the same as normal oxygen, just that it is stored under high pressure. This extremely high pressure turns oxygen into a liquid form.
You can usually find liquid oxygen in canisters, labelled with the abbreviation LOX or also LOx.
Just like with the gaseous oxygen, liquid oxygen also improves combustion in a fire, but it is not flammable either.
However, as LOX is stored under extremely high pressure and in a very concentrated form, it is much more dangerous than normal oxygen.
It can increase the temperature and size of a fire significantly, and although liquid oxygen is not technically flammable, you should use it very carefully.
Firefighters typically know where certain fire risk hotspots are located and this includes liquid oxygen tanks.
These type of tanks are normally located near healthcare facilities such as clinics and hospitals.
They can also be found in many industrial parks which often makes it more dangerous because there are typically other chemicals and flammable materials stored there as well.
Tanks filled with LOX are extremely highly pressurized in order to fit as much of the oxygen in as possible.
Liquid oxygen has a high spillage risk. It reacts quickly with whatever the element comes into contact with.
Typically, liquid oxygen tanks are set into concrete to ensure their stability. However, this means that they are often near asphalt in a parking lot or elsewhere, where they are close to flammable chemicals, such as gasoline.
This combination could quickly combust and ignite if there is no proper precaution and care taken.
As neither O2 nor liquid oxygen are flammable, it is the combination with other elements that makes oxygen so dangerous.
LOX only becomes dangerous when it comes into contact with materials, such as tires, oil, tar, grease, paints and waste containers that can contain any number of flammable materials.
The good news however is that liquid oxygen does not survive in the atmosphere for very long. It typically evaporates very quickly. Still, it can cause serious harm to people and property in its short lifetime.
For example, cars can go through a liquid oxygen spill and add oil, grease and heat to the element.
This can cause sparks to fly or the liquid oxygen to ignite which results in fires almost immediately.
That’s why it is important to report any spills that may come from a tank in a parking lot immediately. This may prevent the risk of fires and any harm to people or buildings.
Conclusion
Despite the many signs warning of flammable oxygen, pure oxygen by itself is not flammable. However, oxygen is an essential component in the combustion reaction of a fire.
The more oxygen is present within a fire, the more it can spread and the hotter the fire gets.
Oxygen is undoubtedly the one element which makes the job of a firefighter so much harder because it is what sustains the fire.
However, when the oxygen supply to the fire is cut off in some way, such as with a fire-resistant blanket or some other oxygen stealing substance or material, then a fire can be extinguished almost instantly.
