I’ve been asked about something that I believe to be a misconception, but I’d like to make sure. The person believes that firefighters stop spraying water on a forest fire once it reaches a certain temperature. The reason given is that the high temperature will cause the water to break down into hydrogen and oxygen, which would fuel the fire instead of putting it out.
I think that it’s highly unlikely this is true. I’ve never heard of anything like it before. I’d guess that water would breakdown at a high enough temperature, but I don’t think that this would be encountered in any practical situation. Does anyone know the temperature at which this would happen? What temperatures do forest fires reach? Thanks
Oh, eventually at a high enough temperature it would dissociate, I’ve no doubt about that. I can’t find a cite for it yet, but I’m sure it would be a fairly high temp, at least several thousands or even tens of thousands of degrees.
H20 is hydrogen which has already been combined with oxygen or “burned.” Of course there isn’t anything quite as pure as H2O, so there could be some impurities in the water, possibly reactive to the superheat from a forest fire. My guess is that it would take at least as much energy to separate the the H2 from the O than the energy released when they were combined. So, no net energy release. IMHO
Water is a product of the basic combustion reaction. Increase the amount of products in your closed system, and you’ve decreased the rate at which the reactants can interact.
You don’t need to guess that it takes at least as much energy to separate the H2 from O than the energy released when they were combined. It must take more energy. Proof:
Let H20 + Q1 -> H2 + O be the reaction for the disassociation of water at high temperatures (FTR, I doubt this happens anywhere near temperatures found in forest fires, but I could be wrong). Let Q1 be usable heat added to disassociate water
Let H2 + O -> H2O + Q2 be your combustion reaction Let Q2 be usable heat given off by combustion
Assume Q1 is less than Q2
Take the output from the first reaction and have it be the input to the second reaction, subtract H20, and you get Q1 -> Q2. As Q1 < Q2, our assumption is invalid as energy has not been conserved.
If the water does disassociate, that reaction consumes more usable heat than usable heat that will be released by the combustion.
As a firefighter, the only reason to stop spraying water onto a forest fire was that there is no chance that the water you have on your appliance was going to be effective in controlling the fire.
Here in Victoria, Australia, we have just come through a relatively busy fire season, and in of the fires that I attended, none of them were hot enough to disassociate the water into Hydrogen and Oxygen. Once a bush fire gets hot enough it can build up a kind of momentum, and start to create it’s own weather patterns. When this happens, there is very little firefighters can do to control this, other than to wait for the fire to reach an area with a lower fuel load where the intensity will be reduced and make firefighting feasible again.
Water can be disassociated through enough heat energy, but only at very high temps. An example of this is the sort of temp reached when metals like Magensium or Aluminium burn. When that happens, (as I know from first hand experience) water applied to the burning metal only increases the intensity of the fire, and can be very dangerous. The only way to put out a combustible metal fire is to smother it with dry sand or dirt.
In short, firefighters don’t stop putting water on a forest fire because it disassociates into Hydrogen and Oxygen. They stop because the water they have is not enough to control the fire.
What’s the stuff in a tree that burns? Cellulose. What’s in cellulose? (Mostly) hydrogen and carbon.
What happens to the hydrogen and carbon in cellulose when a tree burns? They combine with oxygen to form H[sub]2[/sub]O (water) and CO[sub]2[/sub] (carbon dioxide).
So what you see when a tree burns is all the energy given out when water and carbon dioxide are being formed.
If you burn pure hydrogen and oxygen, you can achieve a flame temperature of more than 2000[sup]o[/sup]C (3600[sup]o[/sup]F). That sets a lower bound on the dissociation temperature of water.
In a forest fire, in ideal conditions, you might get localised temperaturues up to 1000[sup]o[/sup]C (1800[sup]o[/sup]F). That’s nowhere near the lower bound for dissociation discussed in the previous paragraph.
The actual total dissociation temperature of water at atmospheric pressure is closer to 4000[sup]o[/sup]C (7200[sup]o[/sup]F).