Apologies if this one has been asked before, but I was unable to find an answer…
Why does water put out a fire? No, really.
Think about it: Hydrogen is combustible. Oxygen fuels fires. Why does water put out a fire?
It can’t be because it’s wet. Gasoline and alcohol are wet, too. Yes,. I know that it’s gasoline fumes that are combustible, not the liquid itself, but never mind.
just a wag, but I think it’s because the heat from most fires is not sufficient to separate water into its components. The properties of electrical fire’s must be somewhat different; note how you are NOT supposed to put water anywhere near an electrical fire, as it only makes it worse.
Water smothers the fire by removing O2 which it needs to burn. If you put a small amount of water on a really hot fire, it will just evaporate and not affect the fire at all.
Free hydrogen is combustable. When it burns, it combines with oxygen to form water. The hydrogen in water doesn’t burn because it’s already burned.
First, I want to address this point:
Free hydrogen burns by combining with free oxygen. So water is basically hydrogen that already burned. Once they’re combined, they don’t burn more. See the words of the allknowing Cecil Water contains hydrogen and oxygen. Why doesn’t it burn?
Second, water puts out fires by dissipating heat. It has a large heat capacity, so it absorbs a lot of calories when it warms up. If the fire isn’t hot enough to warm the water up to the ignition temp of whatever’s burning, it will go out.
Just to add to what Salty and Dr. M said, yes, you can separate water into its elemental components, but this takes as much energy as you got out by burning it. So you can store energy by forming free hydrogen from water, but water will not be an energy source.
As Saltire said, water extinguishes fire by removing heat. Water can absorb ungodly amounts of heat (relative to other materials), its very plentiful, and its nice and cheap, so it makes a very good extinguishing agent. For normal fuels.
If you put plain water on a combustible liquid, be it a hydrocarbon (gasoline) or a polar solvent (alcohol), it won’t work. Hydrocarbons float on water, polar solvents mix with water. Mix the water with a foaming agent, airate it, and you’ve got something that will float on hydrocarbons and not mix with alcohols. But still, you’re using something water based to extinguish the fire.
As for seperating the hydrogen and oxygen in the water. Take a magnesium fire, stand back, and put some water on it. The burning Mg is hot enough to seperate the hydrogen and oxygen, and they then contribute to the fire, generally with some popping and splattering of burning magnesium. To extinguish Mg fires, there are special “dry powder” agents to use, most of them with some type of salt base, if I rememeber right. How often do we come across Mg fires? Most Fords’ steering column housings are made out of magnesium, and you realize it when you hit it with a hose stream. Very irritating.
Geez… it all seems so stunningly logical when explained. I knew I could count on y’all. Thanks!
BTW - I did already know about magnesium fires, electrical fires, oil on water, etc., but failed to mention them. Thanks for further clarifying that
Actually, the reason you do not use water on an electrical fire is the shock hazard. You would probably put out the original fire, but start others due to sparks and fry yourself in the process.
Also, the reason you don’t use water on electrical fires has nothing to do with the fire itself-- It’s because if you’ve got an electrical fire, you’ve got an exposed live wire somewhere, and most water (unless it’s purified in a laboratory) is a decent conductor. It’s the electrical risk you’re worried about.
You might want to check out this thread from a week or so ago:
http://boards.straightdope.com/sdmb/showthread.php?threadid=27102
Once you get past all the Smurf crap, it gets into a pretty heated (yuk, yuk) discussion about whether removing heat can really kill a fire. As Devil’s Advocate, I have yet to be convinced that you can “chill” a fire out… but I welcome a valid proof to contrary.
In my opinion, the old firemen’s claim (endorsed by Saltire & KCB615) that pouring water on a fire snuffs it out by just removing its heat is ludicrous. As if drowning a fire does not affect the oxygen supply or the combustability of the fuel. Please!
Pouring water on a fire prevents the fuel from combining with the oxygen AND lowers the temperature of the material to below its flashpoint (due to water’s EXTREMELY high heat capacity). Both of these properties are what puts out a fire. You could also dump sand on a fire, accomplishing the first part but if the sand got hot enough, it could cause somethinge else to burn, defeating the purpose of putting the fire out in the first place. You want to both stop the flames AND reduce the heat to prevent further burning.
From the previously-cited thread (How small can a fire be?) all the posters pretty much agreed on the following:
A fuel needs enough heat to reach an ignition temperature to START burning.
That fire needs fuel and oxygen to continue burning.
Whether it “needs” heat to continue burning is the unresolved (at least to me) question. I would contend that the fire makes it’s own heat, and you can’t take it away fast enough to put it out.
That said, let’s examine what putting water on a fire does.
First, when sprayed on the flames – like a fire hose does – water chokes off the O2 supply. It does so by filling the air w/ water vapor and screwing with the convection currents that stream fresh, O2-rich air into the blaze.
It also CHANGES the fuel supply… from something that is very cobustible (say, paper) to something not so cobustible, (say, wet paper). I WOULD CONTEND THAT – IN A STRICTLY SCIENTIFIC SENSE – IT IS NOT THE SAME FIRE. Essentially, you’ve switched fuels in the middle of the experiment.
To say that adding water to the paper snuffs a paper-fire by making the resulting fuel harder (if not impossible) to sustain combustion is valid; to say it does so by “taking away heat” is either patently false, or a sloppy, inaccurate description of what’s really going on.
I have a very open mind about this, but as I stated in the other thread, I wish someone would come up with an elegant demonstration – that is not intrusive on the fuel or O2 – that proves that fire needs heat to burn.
Stuyguy says:
Whether it “needs” heat to continue burning is the unresolved (at least to me) question. I
would contend that the fire makes it’s own heat, and you can’t take it away fast enough to
put it out.
So you’ve never blown out a match?
Frolix,
Try this experiment:
Fill your mouth with pure oxygen. Don’t inhale, just fill your mouth.
Light a match and hold it close to your lips.
Compress your cheeks to blow out the match.
Call 911 and ask for the burn unit.
When we normally blow out a match, we starve the flame of oxygen (by smothering it with non-combustible CO2 and by disrupting the normal convection currents around the flame), not lower its temperature.
And, if you don’t mess with the O2 supply in this manner, you can improve combustion – see experiment above; and YOU’VE never heard the expression “fanning the flames”?
To clarify a bit further, if anything in contact with water wants to get hotter than 100 C (212 F), then it takes a HUGE amount of heat to first convert the water to steam. This is because steam has a very high latent heat. To change the temperature of one gram of water by one degree C, it takes one calorie. To change the same gram of water at 100 C to steam at 100 C, it takes a whopping 540 calories.
I don’t think it’s that sloppy to say that the steam generated “takes away heat”, as long as it’s clear that it’s the steam and not the water left over.
stuyguy, you’ve already been presented with a perfectly valid demonstration in the other thread, but you refuse to accept the implications. Triskadecamus described an experiment he did with a Bunsen burner, in which a coil of copper wire was put into the flame and it went out.
The fuel and oxygen needed to support combustion are mixed at the bottom of the burner, so there is no issue with the coil interfering with the mixture. The only thing the coil is doing is absorbing heat.
In fact, this is why the burner doesn’t regularly explode. The metal of the burner itself keeps the gases from heating to ignition until they’ve escaped the tube. The flame can only be sustained above the metal tube, not in contact with it.
Do you have any reason why you won’t accept this proof?
It’s called the “ignition temperature”. “The lowest point at which a fire can begin and continue” (I researched it in World Book).
Okay.
First to Frolix: an apology for my sarcastic tone. No reason for me to have been so rude.
Curt: I totally appreciate what you’re saying and I even understand the basics of the physics and chemistry involved. (And thank God the principle you describe works so well, or half our cities would burn to the ground every year or two.)
But I still say it doesn’t prove your point. Here’s why. My definition of a fuel is a substance that sustains combustion in the presence of the oxygen found ordinary sea level air. The operative words here are “sustains combustion” – that is, it keeps burning.
Under this definition, a cotton rag is a fuel; a granite rock is not. Likewise, a dry newspaper is; a water-logged one is not.
If a wet newspaper is not a fuel, one of MY two required igredients (fuel & oxygen) is missing, so I’m not surprised there is no fire.
I tried two coils, one wound around a pencil as Tris described, and a wider one wound around a 3/8" rod (which seemed it would be a little less stifling). I eased them over the flame, and I dropped them quickly over the flame.
Result: The flame stayed lit in every case!
And here’s my theory (only a theory, mind you) about Bunsen burners. Sure there’s air in the fuel mixture from the bottom vent, but is it enough? Maybe it merely suppliments the air surrounding the flame.
Why can’t someone come up with a demonstration where the ONLY difference between a burning flame and a snuffed flame is the temperature of the situation? No poking wires, no drenching liquids, and no screwing with the fuel or O2 --unless you can provide a control that proves it doesn’t matter.
And you can do ANYTHING YOU WANT with the temperature. ANYTHING! Just keep your hands off everything else.
I mean, isn’t that the sign of a valid experiment: change one – and only one – variable?
(my stressing)
First, normal air is about 21% oxygen. The air you breathe out is about 19% oxygen. Fire will burn quite happily in 9% to 10% oxygen. The carbon dioxide in your breath isn’t going to inhibit that flame all that much. What, exactly, do you think those convection currents are? They’re heat! That same heat that continues the pyrolosis, which therefore continues the fire. Push that heated area away (using the moving air), the fire’s going to go out.
Sure have heard of it. And you know, I even use my breath to get a campfire going. By your logic, that should put the thing out, shouldn’t it? If you blow it too hard, you push the heat away. If you go slow, the air you push in can be used to support the fire. I’m willing to try your oxygen experiment shown above (not with my mouth, however). My hypothesis is that if you push that oxygen fast enough, the fire is going to go out, not get worse. I’ll have to try that soon.