Burning Magnesium Under Water: Mr. Wizard Strikes Out

[tlb]

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.

FYI, I followed all necessary safety precautions: we used welder's
goggles (#10 shade) for eye protection, tongs for handling the
magnesium, and the experiment was conducted outdoors with good
ventilation and plenty of sand for fire safety.

[d***@gmail.com]

Could it be the lack of heat necessary to sustain the reaction?

[tlb]

As I mentioned, I assume that the thinness of the magnesium strip that
I used (even when folded into a four-ply ribbon) is the problem. In
other words, the Mg strip gave up a large amount of heat to vaporize
the water, causing an insufficient amount of heat remaining to sustain
the burning of the magnesium.

I'd appreciate it if anyone who has performed this experiment could
provide me with the dimensions (i.e.., width, thickness, diameter, or
wire gauge) of magnesium that I'd need to use to make the experiment
work.

Thanks again!

[r***@gmail.com]

Post by tlb
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?

If anything, try thinner Mg strips, Mg wire, or Mg powder (try not to
blow up your house!)

Good reading:
http://www.eh.doe.gov/techstds/standard/hdbk1081/hbk1081c.html

[tlb]

Thank you for your link to the DOE site. It's an excellent resource,
but I disagree with your conclusion about using thinner strips of
magnesium. I think that the relevant information is contained in the
first sentence of the "Storage and Handling" section of the "Magnesium"
section of the DOE website:

= = = = = = = = = =
"The more massive a piece of magnesium, the more difficult it is to
ignite, but once ignited, magnesium burns intensely and is difficult to
extinguish."
= = = = = = = = = =

The converse of this statement is also true: the thin strips of
magnesium that I used were very easy to ignite, but they were also very
easy to extinguish when I dunked them under water (most likely due to
the heat transfer issues discussed in previous posts). Low mass and
relatively high surface area make it easy for the strip to gain heat,
but it also causes the burning strip to give up heat easily when it
causes the water to boil (turning water into water vapor).

If I could find a baseball bat made of magnesium, it would probably
burn wonderfully underwater ... however, I might need an
industrial-sized kiln to light the darn thing!

I'm looking for the "sweet spot" where the size/diameter of the
magnesium makes it relatively easy to ignite (e.g., using a bunsen
burner) and will allow it to sustain burning while underwater.

I'm sure that this experiment is performed in chem labs all over the
world ... I wish I had my chemistry professor's email address!

[G. R. L. Cowan]

Post by r***@gmail.com

Post by tlb
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?

If anything, try thinner Mg strips, Mg wire, or Mg powder (try not to
blow up your house!)
http://www.eh.doe.gov/techstds/standard/hdbk1081/hbk1081c.html

The OP blames "the textbooks" for telling him Mg will burn
under water. I had the same wrong idea and tried to light one
end of a Mg ribbon in air while the other end was under water.
The fire always stops when it hits the water

I haven't seen any of the textbooks that say it should.
I *did* see a Poul Anderson story saying this,
and suspect he was having a senior moment,
confusing water with dry ice (CO2(s)).


--- Graham Cowan, former hydrogen fan
Fire without monoxide: http://tinyurl.com/4xt8g

[tlb]

Mr. Cowan:

The U.S. Department of Energy website that was so graciously provided
by ***@gmail.com (post #4) states the following:

= = = = = = = = = = = = = = = =
Extinguishing Magnesium Fires
Magnesium and its alloys present special problems in fire protection.
Magnesium combines so readily with oxygen that under some conditions
water applied to extinguish magnesium fires may be decomposed into its
constituent elements, oxygen and hydrogen. The oxygen combines with the
magnesium and the released hydrogen adds to the intensity of the fire.
= = = = = = = = = = = = = = = =

Many other standard firefighting references ("textbooks", if you will)
recommend against using water (and other aqueous products) as fire
extinguishing media for stopping magnesium fires. The U.S. National
Institute of Standards and Technology deals with this topic in some
depth (at http://fire.nist.gov/bfrlpubs/fire98/PDF/f98052.pdf) stating
that:

= = = = = = = = = = = = =
"Fires in magnesium and magnesium alloys, similar to other combustible
metals, present severe extinguishing problems in that the great
reactivity of magnesium precludes the use of common fire extinguishing
agents. Magnesium has such a strong affinity for oxygen that it
extracts oxygen from oxygen containing compounds. Magnesium reacts
with water to form hydrogen and magnesium hydroxide in such a manner
that the combustion of the magnesium is accelerated, and the hydrogen
burns, supplying increased fuel to the fire. This reaction is
expressed by the following equation:

Mg + 2H2O --> Mg(OH)2 + H2

Because ordinary water based foam agents contain more than 90% water,
they are ineffective in extinguishing burning magnesium."
= = = = = = = = = = = = = =

The site mentions that water is used to extinguish fires in open
burning situations where water can be supplied at sufficient flow rates
to carry away heat needed to sustain combustion. This is consistent
with the previous discussions regarding heat transfer on this thread.

My question remains: what form factor of magnesium would produce
sustained burning underwater?
==>> Thin narrow strips: NO !!!! <<==
I-beams? Baseball bats? Pool cues? Pencils? Or strands the size of
#16 American Wire Gauge?

Any assistance from people with hands-on experience would be
particularly appreciated.

Thanks,
The OP

[H***@nospam.nix]

It isn't going to burn when completely immersed.

The oxygen concentration and the temperature drop too low to sustain
the combustion, unless the ration of magnesium to water is extremely
large, as in a mist, or an unusually large mass of magnesium in a small
pool of water.

[tlb]

Thanks, H. Maybe I'll try repeating the experiment with a crucible
using a high Mg to H20 ratio. This might be analogous to the situation
described in the firefighters manuals, albeit on a smaller scale.

[Spaceman]

"tlb" <***@aol.com> wrote in message news:***@g44g2000cwa.googlegroups.com...
| Thanks, H. Maybe I'll try repeating the experiment with a crucible
| using a high Mg to H20 ratio. This might be analogous to the situation
| described in the firefighters manuals, albeit on a smaller scale.

This is all sorta like trying to light the bottom of a paper cup
on fire while it is full of water.

It will not stay burning unless all the water
is boiled away.
:)

[tlb]

|This is all sorta like trying to light the bottom of a paper cup
|on fire while it is full of water.

|It will not stay burning unless all the water is boiled away.

Hi, Spaceman -

Something like that. When you boil water in a paper cup, it's the heat
transfer from the flame to the water that keeps the bottom of the cup
from igniting. The magnesium is already burning when it's put in the
water, but the energy used in making the surrounding water boil makes
the reaction stop. The situation that firefighters usually deal when
magnesium burns is probably similar to what H was describing in Post
#8: a high ratio of burning magnesium to water (plenty of excess oxygen
to sustain the reaction, relatively little heat carried away by the
water). Firefighters probably don't get the opportunity to totally
submerge a magnesium fire ... they probably don't occur in the bottom
of swimming pools too often!

[Spaceman]

"tlb" <***@aol.com> wrote in message news:***@g49g2000cwa.googlegroups.com...
| |This is all sorta like trying to light the bottom of a paper cup
||on fire while it is full of water.
|
||It will not stay burning unless all the water is boiled away.
|
| Hi, Spaceman -
|
| Something like that. When you boil water in a paper cup, it's the heat
| transfer from the flame to the water that keeps the bottom of the cup
| from igniting. The magnesium is already burning when it's put in the
| water, but the energy used in making the surrounding water boil makes
| the reaction stop. The situation that firefighters usually deal when
| magnesium burns is probably similar to what H was describing in Post
| #8: a high ratio of burning magnesium to water (plenty of excess oxygen
| to sustain the reaction, relatively little heat carried away by the
| water). Firefighters probably don't get the opportunity to totally
| submerge a magnesium fire ... they probably don't occur in the bottom
| of swimming pools too often!

I can see it now.
I grab the phone..
Spaceman : Hello 911, I have a fire in the bottom of my swimming pool.
911 : You have a what?
Spaceman : I have a fire in the bottom of my swimming pool.
911 : What is on fire?
Spaceman : I don't know but i can see it under the water.
911 : click..
silence..
lol

[Dirk Bruere at Neopax]

Post by tlb
|This is all sorta like trying to light the bottom of a paper cup
|on fire while it is full of water.
|It will not stay burning unless all the water is boiled away.
Hi, Spaceman -
Something like that. When you boil water in a paper cup, it's the heat
transfer from the flame to the water that keeps the bottom of the cup
from igniting. The magnesium is already burning when it's put in the
water, but the energy used in making the surrounding water boil makes
the reaction stop. The situation that firefighters usually deal when
magnesium burns is probably similar to what H was describing in Post
#8: a high ratio of burning magnesium to water (plenty of excess oxygen
to sustain the reaction, relatively little heat carried away by the
water). Firefighters probably don't get the opportunity to totally
submerge a magnesium fire ... they probably don't occur in the bottom
of swimming pools too often!

So, does magnesium continue to burn in dense water foams?

[Oscar Lanzi III]

One thing to consider about putting Mg fires out with water: while
immersion in LIQUID watrer will do the trick, a large amount of burning
Mg will BOIL the water from the available heat. And once boiled into
steam, water is going to feed the Mg its oxygen and liberate hydrogen
(boom!).

Didn't I read one time on this NG about putting out a Mg fire and
getting an AMMONIA smell? That's magnesium nitride hydrolyzing, after
having formed by the burning of the magnesium in the NITROGEN of air!

--OL

[John Decker]

Post by tlb
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