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Re: Exploding soda bottles



At 10:31 AM -0500 6/11/00, brian whatcott wrote:

I am unable to decide between the virtues of two possible avenues:

1) Avoid all demonstrations which have some element of risk, for fear of
prosecution (or on a higher level) lest some student be injured.

2) Approach such demonstrations as object lessons in taking sensible
precautions against personal injury.

In the case of the exploding plastic bottle, suitable precautions would
include
limiting the water level in the bottle.
limiting the mass of the CO2 ice.
insertion of the ice by the demonstrator only.
providing a blast shield which could sensibly comprise a 3/4 inch thickness
plate measuring perhaps 2 feet X 1 foot made of transparent plexiglas.
clear instructions to the onlookers to stay behind the shield, ESPECIALLY
if the bottle hangs fire.


I have been doing a very similar experiment off and on for several
years, whenever I have a suitable supple of 'excess' dry ice (we get
it in the finger sized chunks). Sometimes the bio folks have received
a shipment that is packed with the stuff, sometimes we have a bit
left from the cloud chamber set-up.

My method does NOT include ANY water in the 16 oz. bottle, just stuff
as much of the dry ice as you can into the bottle. Do NOT tighten the
cap until you are ready for the explosion!!!!!

I take the loaded bottle up to the observatory platform on our roof.
Set it behind an approved blast shield, but not too close to the
shield. I put the bottle behind the shield at one corner of the
platform. The kids wear goggles and stand way back from the shield.

Unfortunately, just the bare bottle of CO2 takes up to 45 minutes to
'pop' so I now set the bottle into a pan of warm water. This speeds
up the process considerably.

Why is this demo significant??? Well, how many people can say that
they have SEEN LIQUID CO2??

If you DON'T put water in with the dry ice, you are assured that the
liquid that forms IS LIQUID CO2. Several minutes pass while the
contents sit at the triple point, with solid liquid and gaseous CO2
plainly coexisting. I hope to someday mount a suitably chosen
thermistor in the cap and monitor the temp with a cbl.


Our original venture in this area was a two liter bottle stuffed with
as much dry ice as possible, cap screwed tight and then thrown into a
large pond. The bottle sank. Then after a few minutes the bottle rose
to the surface (the plastic stretching enough to become positively
buoyant) then the bottle broke with a satisfying bang and a mushroom
cloud rose while another cloud of fog spread across the surface of
the pond. Needless to say, we DIDN'T see the liquid in this bottle
since it was too far out in the pond.

Recent 16 oz bottles have been set inside a cutoff 1 liter bottle
which is inside a cutoff 2 liter bottle which is nested inside a
cutoff 3 liter bottle. The nesting directs the blast upward quite
effectively and the cap/neck piece 'just' floats back down without
excessive velocity. (I still use the blast shield, even tho the 2 and
3 liter bottles are not ripped by the blast. The 1 liter bottle is
ripped but stays in one piece. The 16 oz. bottle is shredded.





brian whatcott's safety suggestions for the onlookers are certainly
quite appropriate.

(How many of YOU have seen liquid CO2???)

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Chuck Britton Education is what is left when
britton@academic.ncssm.edu you have forgotten everything
North Carolina School of Science & Math you learned in school.
(919) 286-3366 x224 Albert Einstein, 1936