Index Home About Blog
From: sbharris@ix.netcom.com(Steven B. Harris)
Newsgroups: sci.med.nutrition
Subject: Re: Fluoride
Date: 8 Mar 1998 08:22:53 GMT

In <3501C638.1C44@erols.com> physical@erols.com writes:

>My dear,
>
>You are charmingly literal. Do try, however, to read between the lines,
>just a little. You should thus get a big kick out of this: My
>chlorine/fluorine fiasco occured while I was _teaching_ undergraduate
>chemistry! Go figger. Wait 'til I tell you about some of my other antics.
>And what students won't do for a grade... BTW, Group I metals themselves
>do not explode in water. It is the evolved hydrogen gas that ignites from
>the reaction heat and "explodes", but only when the gas is allowed to
>collect in a deep enough beaker, or Erlenmayer flask. If the gas does not
>collect, you just get that neat sparky fizzle.


    Nonsense!  Sodium is close to the density of water.  When placed in
water it evolves hydrogen fast enough that it sits on the water
surface, held up by a cushion of hydrogen gas, much like a piece of ice
on a hot griddle.  The heat is enough to melt the sodium, and thus you
have a rotating molten ball with a thin "frozen" outer surface (where
it has contacted water, then been rotated into the air to cool), held
in shape by surface tension.  This glob sits on the water surface and
sputters and hisses.  For small pieces ( < 1 gram) that continues
without pyrotechnics, until it's gone.

    For larger pieces of metallic sodium in water you are confronted
with the vicious realities of heat transfer and a lower surface to
volume ratio, just as in nuc reactor cores.  A larger piece still melts
and still balls up, but now the sodium is hotter because it cannot get
rid of heat fast enough.  The weight of the ball is also enough to
press it down into the water more, again because the bouyant force of
the evolving hydrogen is a function of surface area (radius squared)
whereas the weight and force trying to sink the ball is a function of
radius cubed.  So with larger pieces, the fractional area of reaction
contact is larger.  The forces of surface tension keeping the liquid
metal in a ball are also not as large in relation to ball size, and if
the glob begins to pancake out, that *also* increases the area of water
contact, and the reaction rate.

    You can see where this leads.  The whole thing, with the
possibility of several disasterous *positive* feedback loops, is
unstable toward the possibility of runnaway.  Too much heat and you get
melting which causes more contact, which releases more heat.  The
ultimate end is caused by bubbles of steam and hydrogen getting under
the liquid molten mass, pushing it out and fragmenting it until the
reaction reaches far higher heats.  At that point it goes BANG in a
classic steam explosion, and throws globules of molten sodium in all
directions.  Where they hit, they freeze in grey stars of oxidizing
metal (mixed with small amounts of lye).  I've had these down the front
of a coat, and thanked diety for my safety shield so they weren't on my
face.  The white cloud accompanying the explosion is steam and lye
powder.   The explosian may or may not ignite the evolved hydrogen.
Often it does not.

    Large pieces of sodium in utterly calm water may explode or not---
or may explode after skating and hissing for a time-- the process is
chaotic and unpredictable.  ANY wave or motion in such systems triggers
immediate steam explosions.   Sodium thrown in streams or brooks will
explode when hitting rapids-- again often not igniting the hydrogen
gas.

     Very much the same happens with potassium, save that it takes much
smaller quantities of metal, and the gas often ignites.

     Rubidium and cesium are reported to explode when added to water in
any quantity.  I've seen these in amps, but never seen anybody with an
amp they were willing to open and fool around with.  So I cannot
confirm.

                                            Steve Harris, M.D.





From: sbharris@ix.netcom.com(Steven B. Harris)
Newsgroups: sci.med.nutrition,sci.chem
Subject: Re: Fluoride [the metalic sodium and water reaction]
Date: 9 Mar 1998 11:32:33 GMT

In <35033CFD.1404@erols.com> physical@erols.com writes:

Harris:
    Nonsense!  Sodium is close to the density of water.  When
placed in water it evolves hydrogen fast enough that it sits on
the water  surface, held up by a cushion of hydrogen gas, much
like a piece of ice  on a hot griddle.

Physical@erols.com:
   >>Firstly, the density of sodium is LESS than that of water,
so it don't need no cushion of hydrogen gas to float, although
the evolving H2 no doubt helps, and aids the scuttling about.
The fact that the sodium MUST float, with or without evolving H2
makes steam explosions not nearly as likely as with the denser
group 1's such as rubidium.<<

    Comment:  Look, the density of sodium is 0.97 (close to that
of water, as I said, and denser than ice by far)-- so if it could
naturally float without any reaction, you'd just see 3% of it
poking up at the surface, like the tip of an apple floating in a
ducking barrel.  That's NOT what you see. Instead, you see this
white, live, hissing, rotating ball of metal on TOP of the water,
like a ping-pong ball gone berserk.  That's from the hydrogen
coming off at an ungodly rate, turning the thing into an air-
hocky puck.  It's not just entertaining, because if you have more
than a gram or two of sodium there, the whole thing is in danger
of going KABLAM with any instability.

   >>Secondly, what you say is likely correct in large enough
quantities, but the technical point still remains that the sodium
itself does not explode, as its decomposition products (other
than H2) do not have greater volumes than the sodium itself.<<


Comment:
    It's a semantic point.  The ball of semi-molten metal will
come apart with a very large bang, scattering globs of molten
sodium everywhere.  I think it's just as descriptive to say that
the sodium explodes.  Yes, I understand that most of the sodium
remains as sodium, so it's not like the explosion of a piece of
Semtex.

  >>Third, what you point out also hinges on the relative
quantity of water, and its pH.  As the reaction goes on, the
water gets more alkaline, making the reaction more and more
difficult.  At pH 14 or so, it might not go at all, although this
is easily verified.<<

Comment:
    It's gunna go as long as there is liquid water.  The only
think NaOH does is gum things up and coat the piece of metal so
the reaction doesn't proceed as fast kinetically.  But it's
nothing to do with equilibrium or pH.  These group I reactions
are very fierce.  Cesium will react with water ice at liquid
nitrogen temperatures (!).  The pH is about as relevant as the
humidity.

	>>Fourth, at the quantities I have used, perhaps the equiv. of
2-3 cc's, molten sodium was not dispersed.  A deranged lab tech
claimed to have thrown a couple of kilograms of Na into a lake,
so his observation should be pretty much definitive on this.<<

Comment:
    I've thrown multi-hundred gram blocks from a 1 kg roll of
metallic sodium, into both lake and stream.  Molten sodium will
be explosively dispersed from pieces much larger than a golf
ball.  And even from thumb sized pieces, if they encounter water
turbulence.  It goes BANG is white clouds of smoke.  The hydrogen
from little pieces sometimes catches fire, and burns fitfully and
with the orange flame characteristic of sodium ions.

  >>Fifth, I have never observed a final water temperature that
would indicate that such steam explosions occur easily.<<

Comment:
    It's a very local phenomenon, caused by water vaporizing in
under and inside the semi-molten sodium mass.  After the
explosion, the water temperature of what's left may not be very
high.  Indeed, I've seen explosions of sodium pieces in very cold
stream water, and I'm sure the stream was heated very little,
overall.

   >>Sixth, take my much embarrassed word for this, impressive
results of any sort do not happen in evacuated hoods, a clear
indication that hydrogen gas is part and parcel of this.<<

Comment:
    I'm not sure what you mean by an evacuated hood (a hood with
good ventiliation?).  Hydrogen may be a factor in the explosions
I've seen from bigger metallic Na pieces, but clearly there is
much steam also, since the cloud produced is white, and contains
a lot of solid reaction product.  Hydrogen, the only other
candidate, disperses very easily, and is not great for creating
shock waves.

    >>Seventh, the sputtering and hissing is no doubt from the
exothermicity I pointed out, but the sparks and popping are due
to the igniting hydrogen, ignited precisely by the exothermic
release. Again, this is not really evident unless the H2 is
allowed to collect.
	The runaway reaction for suitably sized pieces is indeed
plausible, but bear in mind that the melting point of sodium is
LESS than the boiling point of water,<<

Comment: Not by much-- essentially the same.

   >> with a fairly high specific heat, [not compared to water]
so the conditions for this runaway thermal reaction are not met
immediately.<<

Comment:
    Pretty quick!  No matter that shape piece of sodium you throw
into water, you get a sphere from melting within seconds.

   >>  Also keep in mind that the water also serves as a heat
sink, tending to regulate the temperature. <<

Comment:
    Again, unfortunately not.  The sodium is protected from water
contact by a gas cushion, and that both insulates it from heat
loss, and from water contact.  Again, not a stable situation.

   >> The reaction is facilitated by acidic solutions, and perhaps
this is what might have been used in your experiences.<<

Comment:
    Nope-- just plain water.  Some of it quite cold, as noted.

	  >>Rubidium and Cesium, even Francium, exhibit the same
behavior, except Rubidium is the most reactive of them all,
supposedly.<<

Comment:
    And you can bet that nobody has ever collected enough
metallic Francium to see it (and if they did, God wouldn't they
be a fool to try it).

  >>So what exactly was nonsense?<<

Comment: The nonsense was you saying:

   >> BTW, Group I metals themselves do not explode in water.  It
is the evolved hydrogen gas that ignites from the reaction heat
and "explodes", but only when the gas is allowed to collect in a
deep enough beaker, or Erlenmayer flask. If the gas does not
collect, you just get that neat sparky fizzle.<<

   That is nonsense!  Group I metals explode if you put them in
water, in any normal sense of the word explode.   They do it
whether the gas ignites or not.  You can get away with pea sized
pieces, and maybe a bit larger if the water is VERY calm and cold
and you're VERY lucky, but if you use anything else, you are
going to blow you or something or somebody else up.  Maybe not
disastrously like in a Chuck Norris movie, but it WILL go bang.
That's the facts.  I know whereof I speak.

    I'm gunna cross post this to sci.chem, so's I can let a few
chemists over there flame you on this, if they have a mind to.
Just introduce yourself as the guy who says group I metals don't
explode when you put them in water, so long as you allow the gas
to dissipate and not collect.

                                Steve Harris


Let's work on our bedside manner, Stevie.  Maybe your client base
will increase.  Maybe if you are real nice to your patients, they
will live longer--life extension through TLC!
	Anyway, we shall wait 'til said deranged lab tech with the 2 kg
Na files his report.

[He'll see what I've seen.  Do let him read these letters first,
okay?]

 



































































































Index Home About Blog