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Author Topic: Why does water extinguish fire?  (Read 15923 times)

Offline myriam

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Why does water extinguish fire?
« on: 26/03/2010 08:30:02 »
myriam asked the Naked Scientists:
   
hello there

my question is :

Why does the water turn off the fire ?

What do you think?
« Last Edit: 26/03/2010 08:30:02 by _system »


 

Offline Yair Doza

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Why does water extinguish fire?
« Reply #1 on: 27/03/2010 20:12:34 »
it cools it off. Fire require fuel, oxygen and heat. The water cools down the heat stopping the burning reaction of the fuel with the oxygen.
 

Offline Geezer

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Why does water extinguish fire?
« Reply #2 on: 28/03/2010 17:18:36 »
I think that's correct. The water will turn to steam. Converting water into steam absorbs a huge amount of thermal energy (heat), so that helps to prevent heat transfer that will start combustion in more of the fuel. The steam also blocks, or reduces, the flow of oxygen into the fuel. Without oxygen, the fire will extinguish.

If you ever have a fire in a chimney, you might be able to put it out by putting a cup of water into the fire. The steam going up the chimney might put the fire out. However, do not rely on that. Call the fire brigade immediately.
 

Offline myriam

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Why does water extinguish fire?
« Reply #3 on: 09/05/2010 19:03:39 »
yes I agree
this is the macroscopic level
but what happens at the molecular level
 

Offline Geezer

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Why does water extinguish fire?
« Reply #4 on: 09/05/2010 20:00:47 »
yes I agree
this is the macroscopic level
but what happens at the molecular level

Well, I suppose the water molecules get really excited, and some of them get sufficiently excited to go through a phase change from liquid (water) into gas (steam).
 

Offline echochartruse

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Why does water extinguish fire?
« Reply #5 on: 11/05/2010 06:17:23 »
I was taught that water robs fire of oxygen and creates steam
 

Offline Geezer

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Why does water extinguish fire?
« Reply #6 on: 11/05/2010 07:13:21 »
I was taught that water robs fire of oxygen and creates steam

It robs fire of oxygen by creating steam.

The amount of thermal energy required to convert water into steam is considerable, and this cooling effect may be far greater than the deprivation of oxygen.
 

Offline myriam

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Why does water extinguish fire?
« Reply #7 on: 13/05/2010 10:47:59 »
yeah  I agree with this Geezer thank you
and what provokes that sound of tichhhh when we pour the water on the fire?
 

Offline John Chapman

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Why does water extinguish fire?
« Reply #8 on: 13/05/2010 12:09:23 »

If you ever have a fire in a chimney, you might be able to put it out by putting a cup of water into the fire. The steam going up the chimney might put the fire out.


Why would steam put a fire out? Surely once it's undergone it's phase change it can absorb no more heat. Are you suggesting that the proportion of of steam in the air would be so great that the fire is effectively asphyxiated?

By the way, we were always taught in school that water turns to steam at 100c (under normal atmospheric pressure) and so steam is also always 100c. But is that really so? If you put water on a fire that may be 1,000c, for instance, will the water vapour continue to be heated until it is out of the vicinity of the heat source and therefore reach a greater temperature?
 
 

Offline Geezer

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Why does water extinguish fire?
« Reply #9 on: 13/05/2010 17:26:28 »

If you ever have a fire in a chimney, you might be able to put it out by putting a cup of water into the fire. The steam going up the chimney might put the fire out.


Why would steam put a fire out? Surely once it's undergone it's phase change it can absorb no more heat. Are you suggesting that the proportion of of steam in the air would be so great that the fire is effectively asphyxiated?


By the way, we were always taught in school that water turns to steam at 100c (under normal atmospheric pressure) and so steam is also always 100c. But is that really so? If you put water on a fire that may be 1,000c, for instance, will the water vapour continue to be heated until it is out of the vicinity of the heat source and therefore reach a greater temperature?
 

It's a trick an old fireman taught me, so I think it's legit. Water going into the fire will both tend to asphyxiate and cool the fire in the chimney. Some of the rising steam will condense in the air being drawn into the flue. If it's visible, it contains water droplets. The water droplets can absorb heat further up the flue.

The only way to make steam hotter than 100C is to increase it's pressure. In the chimney, the pressure is not going to deviate much from atmospheric pressure, so the temperature of the steam will be around 100C. The actual steam will not be able to take up any more heat, but the water droplets can.

 

Offline Bored chemist

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Why does water extinguish fire?
« Reply #10 on: 13/05/2010 20:31:35 »
Imagine we lived on a really cold planet and, rather than discussing water we were talking about nitrogen.
Would you say "The only way to make (steam) nitrogen hotter than (100C) 77K is to increase it's pressure"?
look back to this planet and we have no difficulty at all getting nitrogen hotter than 77K. Why would water be different?
You can make steam hotter by heating it up.

I can suggest some evidence that water puts out fires, at least in part, by displacing oxygen.
You can't put out a magnesium fire with water unless you flood it with so much that you cool it down. Magnesium carries on burning in steam.

Also, if it was just a cooling effect, then liquid oxygen would put out fires...

« Last Edit: 13/05/2010 20:56:09 by Bored chemist »
 

Offline Geezer

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Why does water extinguish fire?
« Reply #11 on: 13/05/2010 20:52:02 »

Imagine we lived on a really cold planet and, rather than discussing water we were talking about nitrogen.
Would you say "The only way to make (steam) nitrogen hotter than (100C) 77K is to increase it's pressure"?
look back to this planet and we have no difficulty at all getting nitrogen hotter than 77K. Why would water be different?


Bummer! BC is right. I suppose you can superheat steam if it is, for example, surrounded by something at a high enough temperature, so steam can continue to take away heat.

I think my point only applies in the case of boiler that contains liquid water and saturated vapour. Even in that situation you can remove the saturated vapour and superheat it without any increase in pressure.

BC, does the burning magnesium dissociate some oxygen from the water?
« Last Edit: 13/05/2010 20:55:45 by Geezer »
 

Offline Bored chemist

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Why does water extinguish fire?
« Reply #12 on: 13/05/2010 20:56:55 »
Mg + H2O --> MgO +H2
Oh, I forgot to mention, an oxyhydrogen flame burns at something like 2500C. What do you think it produces, and at what temperature?
« Last Edit: 13/05/2010 20:58:27 by Bored chemist »
 

Offline Geezer

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Why does water extinguish fire?
« Reply #13 on: 13/05/2010 21:18:39 »
Mg + H2O --> MgO +H2
Oh, I forgot to mention, an oxyhydrogen flame burns at something like 2500C. What do you think it produces, and at what temperature?

Bloody hot water I'd imagine.
 

Offline myriam

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Why does water extinguish fire?
« Reply #14 on: 14/05/2010 11:17:51 »
hey there

Thank you for all your contributions here
but I wonder if there is a chemical equation between the H2O and the fire
I've heard that the fire could be a sort of plasma

What do you think ?
 

Offline myriam

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Why does water extinguish fire?
« Reply #15 on: 17/05/2010 16:58:01 »
I have an equation

H2O+(excited and restless atmospheric molecules )-> (stable atmospheric molecules) + H2   

who votes for this equation   ;D                               
 

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Why does water extinguish fire?
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