Too Hot Fire and Water

name         Emilie
status       other
grade        other
location     OH

Question -   I have read that water can actually fuel a fire 
that is "too hot".
How is this possible, and how hot is "too hot"?
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Emilie,

This is true, though only certain types of fires get this hot.  One 
type of fuel that can do this is magnesium.  Magnesium burns at 3600 
F (2000 C) and at close distance is brighter than the Sun!  Paper, 
on the other hand, burns at a measly 454 F (220 C).  Since magnesium 
burns so very hot, it can actually split water into hydrogen and 
oxygen (2 H2O --> 2 H2 + O2).  Then the hydrogen and oxygen undergo 
combustion due to the excessive heat to form water again.  While you 
might think that this cycle would keep going, it take an incredible 
amount of energy to split water into hydrogen and oxygen--more than 
you get back out when you reform the water.  All of this heat is 
lost to the atmosphere and the fire will eventually burn itself 
out.  A magnesium fire is so hot that it can burn a hole right 
through a car's engine block (since the engine is made of 
aluminum/cast iron and melt at a lower temperature than magnesium 
burns).  There are other examples, but this is by far the most 
common/popular type of metal fire that water will not work 
with.  Another is called thermite, which is a combination of 
aluminum powder and iron oxide (rust)--this will burn just as hot as 
magnesium.  Lithium, sodium and potassium are all metals that will 
burn and react with water as well.  Potassium is so reactive with 
water that it has to be stored in oil because it will react with the 
moisture in the air.  This is not true of magnesium or thermite, 
which have to be ignited with a very hot flame.

If you have taken any type of fire extinguisher safety courses, then 
you will know that there are 4 classes of extinguishers, A, B, C and 
D.  ABC extinguishers put out most fires with carbon dioxide, while 
class D extinguishers put out metal fires using dry powder.  How do 
they do this?  Table salt!  Believe it or not, there is powdered 
sodium chloride (regular table salt) that comes out of a class D 
extinguisher and puts out the fire by smothering it!  Sodium 
chloride will put out most types of metal fires, though those fires 
containing lithium, it is preferred to use copper to extinguish the 
fire.  There are a couple of others including sodium carbonate or 
graphite, but those have limited uses and drawbacks.

Matt Voss
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Fire is a very uncontrolled process. Nonetheless, it requires 4 
factors: FUEL, OXIDANT, HEAT, IGNITION SOURCE. So "how" a fire 
behaves depends upon the characteristics of these 4 factors, and how 
they interact with one another. With regard to your question:
   If the fuel is a burning metal, the addition of water will 
almost certainly aggravate the fire because most hot metals react 
with water, frequently liberating hydrogen gas which would explode. 
So adding water to any fire in which the FUEL reacts with water 
will increase the severity of the fire. Neither water nor CO2 
should be used against a metal fire -- both serve as further fuel 
and/or oxidant for the metal fire.
   If the fuel is hot enough it is possible that the water would 
turn to steam before it ever reaches the origin of the ignition. 
This hot steam would rise causing turbulence, drawing in more air 
(OXIDANT) from the bottom, making the fire worse. A related effect 
is if the water hits the burning surface, then turns to steam, 
expands, and exposes more fuel to the air. Here, too, the water 
could make the fire worse by exposing more fuel to the other 
components of the fire.
   Yet another way that water can make a fire worse is if the fire 
is the surface of a burning liquid that is less dense than water. 
Here,  even if the fire is not so hot, the water can sink beneath 
the surface of the less dense flammable liquid. The water then 
starts to heat rapidly, but is covered by the burning liquid. If 
the conditions are right the steam formed beneath the surface of 
the burning fuel will "burp", explosively spreading the burning liquid.
   You can see that "fire" is not a simple process, and "pouring 
gasoline on a fire" is not the only way to make it worse.

Vince Calder
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