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Exploding soda bottles



Hello all,

Last year a colleague of mine brought some dry ice into the lab for simple
lab measuring the effects of solutes on lowering the freezing point of a
liquid. A couple of students decided to try an extracurricular excursion,
took a couple of dry ice pellets, dropped them in a 2 liter soda bottle with
some water, tightly sealed the top, and quietly set the bottle in a cement
sink at the back of the lab.

Apparently the effect was all that the students hoped for.

Inspired, this year we ran the lab again, and brought everyone outside to
watch pop bottles explode, from a safe distance of course. A couple of times
the bottles appeared to completely disintegrate, others, we had fragments
that showed the effects of the stress. And that brings about my question.
What goes on at the instant of explosion? The plastic from the bottles that
is left seems to be thicker and less flexible than it was initially. So far
we have 2 theories. One involves the stretching that the bottle undergoes
before explosion, and then the catastrophic release. Does the plastic spring
back and compress? On the other hand, the plastic looks like it might have
been heated. How much of the energy released in the explosion comes off as
heat?

Some warnings. This is a truly violent explosion. The sound of the
explosion is much like a gunshot. (we had a police cruiser drive past the
atheletic fields where we were trying this out) The effect of a soda bottle
blowing up in a hand or next to a body would be pretty severe. Depending on
intitial conditions, after capping the bottle, it may be 15 seconds or it may
be more than 2 minutes before the bottle explodes. The temptation to examine
the bottle is strong if you have been waiting. We once waited several
minutes for something to happen. Instead of grabbing the bottle, we through
rocks at it. All it took was one light touch from the rock, and it went off.
Truly sobering! If you try this, have a plan for how to recover and release
the duds.

A. Njaa
Physics / Chem teacher

"Colors are the deeds and sufferings of light" Goethe