Eggo & Strider: First of all, I don’t think O2 can burn all by its lonesome. Sure, space rockets carry their own (liquid) O2, but there’s also a fuel (liquid H2 or alcohol, if I remember right). After all O2 is an element. What can it do to/with itself – create Ozone? Certainly not break down to Oxygen atoms. If it combines with something, is that not fuel? If not, what’s the burning all about, anyway?
But, I’m much more interested in Strider’s remarks about heat needed to sustain combustion. Basically, my hypothesis is this:
Any fuel that has begun burning is generating enough heat ON ITS OWN to continue burning more of the same fuel REGARDLESS OF AMBIENT TEMPERATURE OR THE TEMPERATURE OF THE FUEL ITSELF; Oxygen, of course, must be present.
IF this hypothesis is true (and, I’m not insisting it is), you can chill the fuel AS MUCH AS YOU WANT but the existing flame will ALWAYS have enough heat to boost the nearby fuel to ignition temperature. Thus it is self-sustaining.
Strider, you’re going to have to prove to me that your chilling of the fuel is strong enough to overcome the heat of the pre-existing flame before I will abandon this hypothesis. And – as my discussion with Tris has shown – just saying so isn’t good enough.
I’ve been lurking in this thread for a day or two now, and I think its time to actually reply to it. I’ll start from the beginning:
Short answer: It depends.
Longer answer: Fire burns mass, and it burns it at a constant rate when all conditions are the same. All fire is is a chemical reaction, if you can slow down the reaction based on enviromental factors (which I’ll explain later), you can decrease the size of the fire. But, it depends on what you mean by “size” and “fire” (if that isn’t Clintonesque enough for you).
Regarding the “size” of the fire: what aspect of the fire are you referring to? Is the combustion taking place with only one molecule, or is it an entire candle burning with a short flame? You could have combustion of one molecule. The fire isn’t going to last very long (since the fire is burning the mass of the molecule at the same rate it would burn a mill), but it will obviously be much smaller in scale. If you want to burn the entire candle with a short flame, now you have to change the enviromental conditions (see below) again.
Now, what are we talking about when we say “fire”? There are two major types of combustion; flaming combustion and smoldering combustion. Flaming combustion has a flame associated with it (duh). Smoldering combustion does not, its mostly a thermal decomposition of the fuel. Smoldering combustion usually occurs in an oxygen deficient atmosphere…there isn’t enough oxygen to react with the fuel and provide the heat to generate the plume that the flame is entrained into.
So the question is, do you want a short flame on a large object, or a normal flame on a very small object? (assuming you even want a flame at all).
Close.
For a fire to start, it needs three things: Fuel, heat, and oxygen. Take a fuel and some oxygen, heat it up enough, it will release combustible vapors (vaporization for liquids, pyrolosis for solids), and the vapors will ignite.
For the fire to continue, it needs four things: Fuel, heat, oxygen, and an uninhibited chemical chain reaction. The heat generated by the chain reaction (reactions between the radicals generated by the pyrolosis or vaporization, initiated by the heat) causes more vapors to be produced, which causes more reaction, etc, etc. So, we have a chemical chain reaction going on that is based on heat flow. Back to those enviromental conditions:
If you can get enough heat removed from the reaction, you can control said reaction to your whim. Heat flows in three ways, conduction (transfer through a solid), convection (transfer through a liquid), and radiation (transfer through non-opaque space). Assuming the fuel is below the flame, most of the heat from the fire will be lost through convection: hot gasses being carried up and away by buoyancy. Radiation and conduction (to a lesser extent) will pre-heat and re-heat the fuel, producing more vapors, and sustaining the reaction. Since heat will flow from the warmer body to the colder body, if you can cause the enviroment to be cold enough, you can suck the heat out of the fire and change its behavior. We put water on it to suck the heat out, if you don’t put enough water on the fire, it won’t go out. If you put enough (too much) on it, it will go out. Apply just the right amount of water to it, and the fire will “stay” where you want it to.
Now for the experiment (you knew one was coming). Its along the idea of the copper-wire-over-the-candle experiment. Take a piece of paper about an inch wide and six inches long, and put a paper clip across it about two inches from the top. Light the top on fire. As the fire burns down, the paper clip will act as a heat sink and suck out the needed heat from the fire, and it will go out.
Now, for the last item.
Nope.
Oxygen is non combustible, it does not burn. Period. It does, however, support combustion really, really, really well. When you hold a flame to oxygen, you’re holding something thats already on fire. The oxygen increases the rate of burning, it doesn’t burn itself. Also, things in the presence of oxygen that don’t burn under normal conditions (ie, steel), can burn quite readily. The Apollo 1 (?) fatal fire is a testament to that fact.
I think that’s my longest post yet…
KC, I’m delighted you’ve come in from the shadows. As you can see, I’m self-appointed Devil’s Advocate, so let me throw a few things out there for you – and whoever else is listening.
It seems that every time someone tries to disprove my hypothesis (above post) by “lowering the temperature” they do so by using – at least what appear to me to be – very suspect, intrusive methods: firemen throw water on the blaze (oh yeah, like all that’s doing is lowering temperature!) (apologies for the sarcasm), Tris wraps it in a coil, and you impede its progress with a paper clip.
I realize that my kitchen is not Bell Labs, but can we devise a more subtle – and convincing – experiment?
Regarding the experiment you suggested in your post, I tried it, and sure enough it worked like you said it would: flame died when it came to paper clip.
But then I tried it with a twist: I used a pc that was about 1/2" longer than the width of the paper strip, so it stuck out on one side. I held the sticking-out end in the lit gas burner of my stove for about 10 secs. to get the whole pc good and hot. Then I lit the top of the strip and watched it burn – still holding the end of the pc in the gas flame.
No doubt you can guess my logic here: if the pc isn’t cool, it won’t draw off any heat and thus should not impede the flame. But, as you can also guess, the flame went out.
Short of a really good freezer (I’m going to take a guess and say around -150F or so) and some very accurate controls for it to tune it to the exact temperature that would form an equilibrium with the fire, I’d say we’re not going to devise anything. Its kind of a narrow window you’d have to hit to get the desired effect, and you’d have to have a very predictable fire…fuel and oxygen would have to be very consistent. A stiff breeze would probably accelerate your fire beyond your temperature control device.
In regards to the paper clip experiment, now you’ve got me thinking (that’s scary stuff in and of itself). I’d say the paperclip probably wasn’t quite hot enough to not draw the heat out of the burning paper, but thats speculation right now. I’m going to have to crunch some numbers (damn you) and figure out exactly how much heat is lost to the paper clip vs the heat generated by the burning paper.
OK, the paperclip thing, IMHO, is not a very good experiment because what the paper clip is doing is stopping the progress of the flame with an obstacle. KCB615 said:
“For the fire to continue, it needs four things: Fuel, heat, oxygen, and an uninhibited chemical chain reaction.”
Well I would say the paper clip is inhibiting the chemical reaction by getting in the way. The temperature of the paper clip has nothing to do with stopping the fire.
Now about Oxygen. Lets say I open up a tank of oxygen. I now have a room full of 75% oxygen. Now I light a match. What will happen?
I have been taught that oxygen is flammable. Thus when I light a match it acts as my ignition source and ignites all of the oxygen in the room with a big bang.
According to what you say, the match flame will get really big but that is it. So then oxygen is not flammable?
and finally…
stuyguy:
You have been playing devil’s advocate against EVERYONE else, saying until all of us prove you wrong, then our arugments are wrong. Well why don’t you go look this up yourself and tell us we are wrong. Everyone here has done their part in trying to convince you, and you keep saying no. I am seriously not trying to be mean, or bitchy or anything like that, but before you tell everyone else to go get evidence to prove you wrong, why don’t you get evidence to prove you are right. Again, I am sorry that I am coming off like an asshole, but I felt that it needed to be said.
Strider: first of all, I’m not offended by your tone. Getting a little carried away is part of the fun.
Besides, you made a good point… I should be contributing a little more to the practical aspects of this discussion. I have, however, been giving this LOTS of mental attention (too much, in fact) and it’s time I shared my thoughts with fellow posters.
In my opinion, here’s what constitutes a “fair” experiment:
Nothing must touch, get very near, or interfere with the flame itself, the fuel or its delivery system, or the 02 delivery system.
The quantity and quality of the fuel and O2 must be essentially unchanged from heat-present to heat-absent conditions.
(I’m unsure about this one, 'cause I don’t know if it’s “fair” or not…) The physical state of the fuel must be unaffected by the change from heat-present to heat-absent conditions. (Here’s why I’m troubled: You’ll remember I gave Tris grief about his camp stove scenario… I still think I was justified in charging that the chilled, less-viscous fuel wasn’t reaching the flame; but – assuming a mechanical delivery system is not an issue – would I be justified in claiming that if the fuel thickens or freezes it’s not really the same fuel? Hmmm… I’m not sure. There is no chemical change, but there sure is a physical one; and does that INHERENTLY affect the fuel delivery system – even a “passive” one like in a wickless, burning puddle of oil? Don’t know.)
Some practical, random thoughts that might make the demonstration easier to perform and/or more convincing:
The temperature differential we create should be stark and active. We should “add cold,” not just “take away heat.” (STOP! I KNOW YOU CAN’T REALLY ADD COLD. Cut me a little layman’s slack in my terminology…) Ambient room temperature – as in the coil and paper clip tests – should not be our heat sink if we want a convincing demonstration.
Without lab facilities, our best option is probably to chill the fuel, rather than the surroundings. In which case…
We should probably opt for a liquid fuel; it (I think) chills more rapidly and thoroughly than a solid, and – unlike a gas, which must be pumped – it can be made to burn passively (at it’s own rate, and acting as its own delivery system).
We should choose a fuel that burns at a naturally low temperature so as to make the task of snuffing it out as easy as possible.
Whew.
Now… all the above thoughts have led me to propose the following experiment (your suggestions are welcome):
Take a small vessel that’s fireproof and conducts heat well, like a metal tablespoon or small aluminum pie plate; fill it with a shallow puddle of a liquid fuel that does not burn too hot or too high (an oil perhaps); set it on fire; dip the underside of the vessel into ice water; see what happens!
That’s the best I’ve come up with so far. I have not tried it, but would love to hear everybody’s thoughts (especially about fuel suggestions) before I do.
Thank you stuyguy for your understanding, it felt like everyone here was arguing with a wall.
As to your experiment let me say this. I agree with KCB615 that we do not really have the facilities to make an experiment. In your experiment we are taking the heat out of the fuel, not out of the fire. What we would need is a chamber that can go down some serious negative degrees otherwise, anything we do is in vain. I have been researching on the web and can not find a website that descibes the basic needs of a fire, but I will prevail!
The reason that I believe your experiment, and almost any experiment we, as laymen, can preform, is flawed is because of the nature of what we are looking at. The whole essence behind what we are saying is that if you have a fire, no matter the substance, and you take the heat away from that fire, then it will die. Simply chilling the fuel, or putting an obstacle in the way does not demostrate what I am talking about. If we surround a fire with cold air, the fire will lose energy. Why? Because not only is the fire expending more energy to heat up the fuel that has been chilled by the cold air, the fire is also losing more energy through convection, the fire is expending energy into the air around itself. If we push the temperature down enough, the fire will expend more and more energy to the cold air around it and eventually will not have enough energy to heat the fuel to the combustion point.
I am not trying to sound sarcastic, but what is your arguemnt against this?
Consider the following totally hypothetical situation: A lump of carbon in the depths of space exists at absoute zero (or as close as you can get) and is surrounded by oxygen which we will say is also at absolute zero and is therefore solid or liquid or whatever. A mystical power (bear with me) then raises the temperature of one single carbon atom until it acquires the activation energy to react with one O2 molecule and produce CO2 plus a certain amount of heat and light.
The question is, under these impossible conditions, will the heat of reaction raise the local temperature of at least one of the neighbouring atoms such that a second reaction takes place? OR… will the energy be dissipated so widely that the system just gets hotter. If the latter case is correct, then yes you can freeze out a fire. If the former, no you can’t.
I don’t know chemistry and I don’t know the answer, but I think this is close to the fundamental question.
Geez Strider, we’ve got to limit these discussions to normal times… these late hours are wrecking my sleep cycle.
I have no idea if my tablespoon-of-burning-fuel-dipped-in-ice-water (let’s call it TOBFDIIW to save me the typing) idea will work, but I based it on the assumption that Tris’s and KC’s experiments were ulimately valid, and just their methodology was dubious. IF those experiments did, in fact, kill the fire by drawing away heat with just a paper clip (!) or wire coil (!!), it seemed that we wouldn’t need a tank of liquid helium – or a glacier – or absolute zero – or the depths of space – (you get my point) – to accomplish the same thing.
Admittedly my TOBFDIIW makes no attempt to chill the flame itself or the air around it – like you seem to prefer. But I think it would be more effective to put the cold where the rubber meet the road: at the chemical interface between the flame and the fuel. (That, incidentally is why I advise a fuel with a low-to-the-ground flame that burns at a fairly low temperature.)
I did devise another experiment (also untested) along the lines you suggest, but did not propose is because of it’s complexity. (Complex, at least, when all you’ve got is a household kitchen at your disposal). I came up with it in response to Tris’ coil experiment…
I’m sure we’ve all seen candles that get blown by a breeze but did not go out; I, in fact, would sometimes lightly blow on a candle flame to see how long I could make it struggle to hold on to the wick before it died. Well then, you should be able to aim a light, steady room-temp stream of air at a flame for, say, a minute, and have it not go out (the control); then if you suddenly chill that air, it should go out (the experiment).
I’m not crazy about this idea – I think it’s pretty dubious, myself – but if Tris’ coil could snuff a flame by lowering the air immediately around it to just room temperature, this experiment should work too.
I’ve GOT to get some work done today, so I’ll leave my responses to SM’s post for later (or to another poster).
To paraphrase, you’re saying that the paper clip is acting as a fire wall, seperating the two portions of the fuel (paper) from burning. Its a valid idea, but the paper clip doesn’t seperate the fuel. There is still a continuous piece of paper weaving through the paperclip. Also, at least one side of the paper is always left exposed, the paper clip doesn’t clamp down on both sides of the paper. That way, oxygen can still get to the fuel, and there is room, so to speak, for the reaction to take place.
The room won’t go boom, but that match will burn like you never thought possible. In a 75% oxygen atmosphere, you probably won’t be able to hold onto the match for all that long. Think blowtorch.
Nope. Completely nonflammable. You can heat oxygen up as much as you want, it will never ignite. If you take another gas, such as propane, methane, or carbon monoxide, if you heat them up enough, they’ll ignite (920, 900, and 1128 degrees F, respectively). You have to have enough of the gas there to ignite (upper and lower flammable limits), but it will ignite. Oxygen won’t. It only helps other materials burn.
Exactly. We did this experiment in my 10th grade chemistry class (more or less). We filled an inverted jar with oxygen from some chemical reaction (or possibly eletrolysis, I can’t remember), and put it over a burning birthday candle. The flame got really bright, and the whole candle burned in seconds.
Oxygen is an accelerant. The exothermic reaction is the oxidation of the fuel; more oxygen means more oxidation. The potassium nitrate used to make fertilizer bombs works because it releases oxygen when heated, thus accelerating the burning of the fuel oil.
Which reminds me: I saw a Mr. Wizard episode once (talk about pyros!), where he sprinkled some potassium nitrate on a piece of steel wool and lit it with a fuse. Whooosh… instant pile of powdered rust. The same experiment worked really well with a bowl of cornflakes, as well.
Two comments about oxygen being flammable. Most oxygen we encounter is molecular oxygen or O2 (sorry I don’t know how to do subscripts). There is a substance called ozone which is (I think!) O3. Could not this be called burned oxygen?
Also, I read somewhere (perhaps “Noble Gases” by Asimov) that if you have an environment of Fluorine gas you can get stuff to produce a flame in it. Perhaps this is not “burning” depending on your definition of “burning”. But there will be a flame. The kicker is: even a jet of oxygen will “burn”, producing a fluoride of oxygen, not an oxide of fluorine.
So to those of you that actually know something about chemistry: do either of these facts mean that, in some sense, oxygen can burn?
This answers the whole question of whether heat is required to sustain a fire, for me. Do you find fault with the above logic, stuyguy?
As for the smallest fire: Under the right circumstances, if you had a chain of molecules sitting end to end, they could continue the combustion for as long as the fuel was available. But you can’t roast marshmallows over a single, burning molecule, so who cares?
Correct. Oxygen is not flammable. Combustion is the reaction of a fuel with an oxidizer (normally oxygen). Oxygen cannot combine with itself–it’s already combined. The match will burn fast and hot, but once it’s gone, the combustion ends (assuming there’s no other nearby fuel source).
I used to work in a welding supply center, and we “experimented” with the various gases that came through our center.
Regarding the heat experiment, Here’s my submission:
We need to use gaseous fuel, since only a gas can actually burn. Any solid or liquid at low enough temperature will not give off vapors necessary for burning, making it a fuel starvation issue instead of a heat one.
Suppose we have a bunsen burner or a welder’s acetylene cutting torch with the nozzle extended AFTER the oxygen inlet. [list=1][li]Around the extended nozzle, we place a switchable refrigeration coil (maybe we run liquid He[sub]2[/sub] through the coil). []We light the torch, let it burn a few minutes and optimize the burn (activate the oxygen feed lever on the welder’s torch, open the inlet on the bunsen burner). []Then we activate the refrigeration coil, thereby chilling the pre-combustion fuel/oxygen mixture.[*]Hilarity ensues. We watch the burn and see what happens.[/list=1][/li]
Actually, I have a (sort of) working model of this experiment and use it every day: It’s called an '83 Mustang. Carbureted, not injected.
On really cold winter days, I have controlled combustion taking place inside very low temperature iron chambers. When the temperature dips, the engine is very difficult to keep running. Although I think this may be a fuel starvation issue as well, since the problem is most likely failure of gasoline to vaporize in the cylinders. Maybe if somebody has a CNG or LPG vehicle? In Canada? hmmm…
Remember the show: John Davidson, Fran Tarketon, Cathy Lee Somebody.
Well, IIRC they did a piece on a technology to reduce the likelihood of fires/explosions associated with automobile gas tanks (probably this was during the Golden Pinto Era).
What they presented was a loose wire mesh insert that occupied the entire volume of the tank. Since was a mesh, it didn’t take up that much space and gasoline, as a liquid, easily flowed through the openings in the mesh.
They showed the dramatic effects of firing a rifle round into a normal gas tank filled with gasoline-no vapor space. It went BOOM. The heat generated by the impact of the bullet resulted in a catastrophic fuel and oxygen consuming failure. Then, they targeted a second full tank with this wire mesh insert in place. A small flame that quickly self-extinguished was the only result. Same heat input, but no big explosion. Why, rapid conduction of energy away from the bullet’s point of entry, starving the reaction of heat.
Grumble, grumble. Should have read, “Why? The rapid conduction of energy away from the bullet’s point of entry starved the reaction of heat.”
BTW: I have access to -80[sup]o[/sup]C and -140[sup]o[/sup]C freezers. If someone can suggest a safe experimental design, I’d be willing to try a burn in them.
Random comments:
A fire needs heat to start. The best way to learn this is
to try to start one in the snow!
Oxygen itself is not flammable. Flammable means “easily
capable of burning with flame”. Oxygen does not burn, it
is the other materials (usually carbon-based) that, in
combining WITH oxygen, release heat and gases. The flame
is heated gases emitting a characteristic spectrum.
Sodium, magnesium, hydrogen, and powdered aluminum are some other examples of flammable elements.
Flies and other insects may have a different sense of
time than we do. I don’t know that synaptic response and
other chemical responses happen at the same rate in other
species as in humans.
A millimeter tall person lighting a match would see it burn out fast, just as we see the match burn out fast. That’s assuming that we both have the same subjective experience of time. On the other hand, if I were that millimeter tall person I’d use a match that was small by my standards. The normal match wood would be enormous. If I lit a “small” fire on one end, it wouldn’t burn very fast. The millimeter me experiencing a match lighting would see a big explosion followed by the match wood being engulfed in flame.
Fascinating, fascinating thread. This kind of stuff is what brought me to the SDMB in the first place.
I’m not a physicist or chemical engineer, but I wanted to comment on something that struck me as illogical. In proposing the parameters for a possible experiment, stuyguy included this:
Seems to me that if you did this, you’d have to have a really cold chamber to reduce the heat to a point where it affects the reaction. In other words, you look at the differential between the combustion temperature and the extinguishing temperature. If you have some sort of chemical burn that ignites just above room temperature, you’d have to bring it way, way down to snuff it, beyond the capacity of any equipment you’re likely to find in stuyguy’s Idealized Gedankenkitchen.
Given those limitations, it seems to me you’d want to design the experiment to utilize a flame/burn in a middle range that can be reduced to the snuff point by common household devices. Too hot, obviously, and you can’t handle the reaction safely. Too cool, and you can’t get it down far enough. Seems like the Goldilocks Principle in action… or am I off base?
Perhaps not shedding much light on the query, but…
Smurfette was made by G. She originally had dark hair(becse she was evil!) but Papa Smurf, like a true evangelist, got the evil out. So then she’s blonde. There’s a moral in that somewhere.
Onto fire.
My suspicions come in many ways.
One is from suspicious mind: would we need a perfect heat sink in order to remove enough heat to stop combustion?
One is from no one in particular. If the environment was cold enough my last suspicion is defeated since most environments are terrible heat sinks. Thus, the oxygen doesn’t get there fast enough because of insufficient convection.
But then this brings up the fuel source issue.
I agree ith K, though…HALON systems stop the reaction. but doesn’t it stop the reaction by using up the oxygen? That doesn’t make it really a fourth condition for fire (heat, oxygen, fuel).
sigh
All I really added was a comment on the smurfs, long dead in this GQ.
The Halogenated agents (and dry chemical agents, for that matter) interrupt the chemical chain reaction. They don’t do anything to remove the oxygen from the fire (at least in a smothering way). Carbon dioxide works that way, but not the halons.
The halons do remove some of the oxygen, but only after its been ionized. Same with the carbon, hydrogen, and hydroxyl ions floating around in the reaction zone. The halons are super-reactive, so they suck up the free radicals and neutralize them before they can continue the combustion.
