Re: Burning Magnesium Under Water: Mr. Wizard Strikes Out
- From: John Decker <John_member@xxxxxxxxxxx>
- Date: 8 Feb 2006 21:40:28 -0800
In article <1138635301.972442.86950@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, tlb says...
----------------------
In response to a question from my son, I recently attempted to
demonstrate that magnesium will burn under water - just like it says in
the textbooks. I used a thin strip of magnesium (one eight inch width)
which ignited readily in air with a sustained flame. When I placed the
Mg strip on the surface of the water, it continued burning vigorously,
causing some localized boiling of the water. However, when I plunged
the magnesium strip completely under water, the water extinguished the
flame ... every single time.
Needless to say, the results of this experiment have somewhat tarnished
my reputation as Mr. Wizard. I assume that the reason the flame went
out was because of heat transfer effects related to
dissociating/vaporizing the water (I tried folding and twisting the
magnesium into four-ply strips, but that had no effect).
Could someone tell me what I need to do to make this experiment work?
Would using thicker magnesium strips help? If so, what size would I
need?
Thank you in advance for your help.
Water on Magnesium Fires...
"Although water in small quantities accelerates magnesium fires, rapid
application of large amounts of
water is effective in extinguishing magnesium fires because of the cooling
effect of water. Automatic
sprinklers will extinguish a typical shop fire where the quantity of magnesium
is limited. However,
water should not be used on any fire involving a large number of magnesium chips
when it is doubtful
that there is sufficient water to handle the large area. (A few burning chips
can be extinguished by
dropping them into a bucket of water.) Small streams from portable extinguishers
will violently
accelerate a magnesium chip fire.
Burning magnesium parts such as castings and fabricated structures can be cooled
and extinguished
with coarse streams of water applied with standard fire hoses. A straight stream
scatters the fire, but
coarse drops (produced by a fixed nozzle operating at a distance or by use of an
adjustable nozzle)
flow over and cool the unburned metal. Some temporary acceleration normally
takes place with this
procedure, but rapid extinguishment follows if the technique is pursued.
Well-advanced fires in several
hundred pounds (100 lb equals 45 kg) of magnesium scrap have been extinguished
in less than 1
minute with two 37.5 mm (1-1/2 in.) fire hoses. Water fog, on the other hand,
tends to accelerate
rather than cool such a fire. Application of water to magnesium fires must be
avoided where quantities
of molten metal are likely to be present; the steam formation and possible
metal-water reactions may
be explosive."
http://www.eh.doe.gov/techstds/standard/hdbk1081/hbk1081e.html
JD
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