The FIRE TRIANGLE: Very oversimplified??

Everyone who has ever talked or written about fire, brings up the “Fire Triangle” - especially firemen.
The fire triangle basically says you NEED three things for a fire to exist: Heat, Fuel, Oxygen.

The main example references burning wood or coal or something. There is an initial heat source, the coal or wood is the fuel, and there is oxygen supplied in the air. Take out one of these from the equation and you no longer have a fire. But is this really a universal truth? It sounds childish to me.
Is Oxygen really necessary??? Aren’t other gasses flamable?
Couldn’t I sustain a flame on just hydrogen. Sure, it’s hydrogen and heat, but the triangle says I NEED oxygen. Do I really need oxygen present for the hydrogen to burn? What about sustaining a fire with just pure oxygen. What “fuel” is there? I have heat and I have oxygen, but no fuel.

If the fire triangle is this ridiculous, why is it so popular??

Yes, you still need the oxygen. I remember a joke/riddle that asked where the safest place to stand in a room was if the room was filled with 100% hydrogen and you lit a match. The answer was, it wouldn’t matter, without oxygen there would be no explosion.

Not only do you need oxygen but you need fuel and spark the triangle stands. If you could have combustion without all three then rockets wouldn’t need to carry both fuel and oxidizer they could carry just oxygen.

Yes you need oxygen, since burning is the combining of a material with oxygen.

Actually, though, you can wriggle out the “heat” side of the triangle: we make pyrophoric material at work (it is a reduced chemical), and if you expose it to oxygen it will spontaneously burn. Of course, the trick here is that the heat of activation is extremely low, so even the ambient temperature in a freezing warehouse is more than enough to set the reaction off. So, it doesn’t need heat in the common sense of the word, although a picky person will point out that we’re cheating a bit here.

The thing is, this does make the “fire triangle” impractical for fire-fighting purposes, which is the whole point of it. If you try to fight this type of fire by removing the “heat” side of the triangle - i.e. by spraying water - you will only make things worse as you scatter fiercely burning powder all over the place.

There’s a common misconception that oxygen “burns” and that a room full of pure oxygen may as well be a room full of jet fuel fumes: one spark and KABLAMMO!

Oxygen doesn’t actually burn at all, but its precense allows OTHER subtances to undergo chemical change when the heat is high enough. Oxygen is (by far) the most common of a class of chemicals known as “oxidizing agents” that act as catalysts for exothermic reactions (i.e. burning, but also rusting). Put simply, when something gets hot enough, its molecules want to break apart. Think of this as the “Spice Girls” cohesive principle. At first, the configuration seems stable. Eventually, when the heat climbs high enough, elements want to break away, but only if a better cohesion (i.e. a promising solo career) offers itself. An oxygen molecule is a pretty tempting offer, since it adheres quite easily to almost anything. The very act of a molecule splitting up and recombining with oxygen can generate a lot more heat (most reactions of this type are exothermic: think of it as press buzz when a group member goes solo, creating a little extra spark of public interest). If there is no free oxygen around, a molecule may as well stay the way it is, cranking out bubblegum music for years until everyone gets sick of it and it gradually decays.

You are warned not to smoke in an oxygen-filled room not because the oxygen itself will ignite, but because it’s presence makes it easy for other substances to ignite. As stated above, oxygen is the most common oxidizer, but it’s not alone. It is just the one you are 99% likely to encounter in an office or house file. Take away the oxygen and the fuel can no longer have any exothermic reactions. The heat drops and the fire goes out.

If it was that ridiculous, you’d have a good question. But since it’s not ridiculous, what’s your point?

In another thread recently, somebody said that water ‘burns’ with fluorine (although of course, water contains oxygen, so maybe it’s just a special case).

You can have fire(of a sort) without oxygen; the sun is a good example of this, but runaway fusion isn’t what firemen generally find themselves dealing with; the fire triangle is pretty sound - firemen usually work on removing one or more corner(s) of the triangle:

Remove heat: Water cools the fire, preventing it from sustaining itself - another way to cool a fire is to disperse the burning objects - you can see this for yourself if you remove a glowing coal from the barbecue; it will not burn for long away from the sustaining heat of the heap.

Remove Oxygen: smothering the fire with a flameproof blanket or with a layer of foam, or displacing the oxygen with a non-flammable gas like CO[sup]2[/sup] or Nitrogen - fire is essentially rapid oxidation; remove the oxygen and the process cannot continue.

Remove Fuel: Turn off the gas and the flame goes out - other fuel sources can be difficult to remove from the fire, this might be why firemen generally tend to concentrate on the other two corners of the triangle.

How do these things apply to the Apollo 1 capsule? Was the fire actually burning along the surfaces inside while the oxygen rich atmosphere fed it?

In an oxygen-rich environment, all sorts of things that we commonly regard as non-flammable will burn quite merrily; including metals (small pieces of metal with a high surface area-to-volume ratio (like steel wool) will burn in normal air but bigger pieces won’t, they will if provided with more oxygen).

Some things that we regard as moderately flammable in air(plastics, fabrics) can burn explosively in pure oxygen.

Yes. Another term that can be used for what you’re describing is oxidation.

Yes, meaning they can oxidize, or burn with oxygen. Oxygen is not flammable.



You will not have, nor sustain, a fire with just pure oxygen. If you have a fire, you have a fuel, which is being oxidized by the oxygen.

It’s not the fire triangle which is ridiculous, but your notion that fuel without oxygen, or oxygen without fuel, can yield fire. Sometimes the fuel source or the oxygen source isn’t painfully obvious, but you can be sure that they’re both there.

What gave rise to this question? What has led you to believe that there was fire without fuel, or fire without oxygen?

The Apollo 1 capsule was full of velcro strips, nylon webbing, plastic hoses, rubber gaskets, and insulated wires. All of these materials had been tested for reasonably low flammability under normal atmosphereic conditions, and under the low-pressure pure oxygen that the capsule would have held in orbit. Under pure 14PSI oxygen conditions (which were only tempoary while they worked the bugs out of the capsule on the ground) they were all much more flammable - the velcro in paricular would burn like flash paper with that much oxygen available.

What killed the Apollo 1 crew wasn’t so much the flames or the heat, as the fire pulling all the oxygen out of the air and replacing it with toxic fumes. Had the capsule not been full of flammable materials, they would have been fine.

But you don’t necessarily need free-standing O2 to start combustion. Other combustible combinations include N2O4 and UDMH, nitric acid and kerosene, and millions of other combinations. Some combinations do not require a spark at all; they react when they come into contact with one another.

From my chemistry book:

No oxygen.

OK, OK, the section starts off with “Chlorine is less active as an oxidizing agent than flourine”, so you could argue that in this reaction Chlorine is the “oxygen”, or better that the “Oxygen” corner is really short for the “oxidizing agent” corner.

The hydrogen/chlorine reaction may be exothermic, but it won’t really count as a “fire” unless it can ignite the matter around it, i.e. it spreads beyond its initial source. If there’s no oxygen (or other oxidizer) nothing else will burst into flame and once all the free hydrogen and/or chlorine is consumed, the heat source dissipates.

One thing missing from most fire triangles are the “Free Radicals of Combustion”, without which, fire doesn’t work. Think of them as the glue that holds the sides of the triangle together. Some fire-fighting agents suppress the free radicals, and thus stop the fire, even though oxygen, fuel, and an ignition source are still present.

Actually, fire classes now teach the fire tetrahedron:

It’s basically the same as the fire triangle, but with the addition of the chemical chain reaction so that it’s technically correct. I think that would be the same as the Free Radicals of Combustion that Tranquilis referred to.

Oh yeah, here’s a nifty link.

Also, the change from O[sub]2[/sub] to oxidizing agent means that chlorine, fluorine, and other oxidizers could function in place of oxygen.

So to answer the OP, the fire triangle was a little simplistic, but it’s still a good model for most of the fires we fight, so it’s certainly not ridiculous.


This is also where halon fire extinguishing agents do thier work, they interrupt oxidation reduction reactions.

One of my fire academy instructors used to always love to tell us:

"you need 4 things for a fire, heat, oxygen, fuel, and a battalion chief. Take away any one of the four and the fire will go out.:smiley:

I’ve never heard of such a restriction. What do you base this statement on?

My chemistry book [1] doesn’t list “fire” in the index, but it has this to say about combustion (bolding mine):

Any definition for fire I’ve seen defines it in terms of burning or combustion, so I believe hydrogen burning in an atmosphere of chlorine is “fire”.
[1] College Chemistry with qualitative analysis (sixth Edition) by Nebergall, Holtzclaw, and Robinson, Section 9.5.

I discount the reaction as a “fire” in the sense that it isn’t something a firefighter or a person wielding a fire extinguisher will have to deal with. The hydrogen is “burning”, fine, but in an oxygen-free environment, it won’t set anything else on fire. In combatting a fire, the greatest concern is to keep it from spreading, and spreading isn’t going to happen in the very specific conditions described in the hydrogen/chlorine experiment.

If I strike a match, I’ve made a short-lived fire that will die as soon as the fuel (the unburnt paper or wood of the match itself) is consumed. But if the still-burning match falls on a pile of paper or wood shavings, then I have a self-sustaining reaction that may eventually grow out of control, requiring me to examine the fire triangle and decide how to stop it.

It’s really nothing more than a chosen definition of the word “fire”. I know that in the broadest sense, it applies to all forms of combustion (and, one could argue, most nuclear interactions) but for the purposes of this thread, I’ll focus on “fire” as a large-scale process that may require grabbing the extinguisher or calling 911.