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It went boom November 27, 2011

Posted by mareserinitatis in education, physics, science, teaching.
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A few years ago, I was working on as part of an NSF educational program.  The idea was to pair students (undergrad and grad) with local science and math teachers to help them do whatever they needed.  I was paired with a physics teacher who asked me to build a Ruben’s tube.  (I suppose I should put the instructions here some day…they’re still on my old blog.)

If you’re not sure what I’m talking about, here’s a video of the demonstration.  I’m holding the speaker at the end of the metal tube that’s covered with saran wrap.  The noise from the speaker is vibrating the saran wrap and hence the gas inside the tube.  At the right frequencies, you get standing waves, which is visible in the flames coming from the top of the tube:

Anyway, as a safety measure, we started by throwing some dry ice in the tube.  This was supposed to evacuate out any oxygen in case the flakey methane jets in the high school lab went out.  (Carbon dioxide is heavier than oxygen, so it’ll stay in the tube and force the oxygen out.)  If the jets did go out with oxygen still in the tube, then I’d be standing at the open end of a pipe bomb because the flame would’ve backed into the pipe and ignited all the methane inside.  Without the oxygen, there would be no explosion…just a slow burn off of the remaining methane.

In order to test this, I got a bunch of dry ice from the chemistry supply at NDSU.  I filled a couple of soup thermoses and brought it to the high school.  We tested things out, and our first test went fairly well.

As I was leaving the school, I was walking toward a teacher in the hallway.  I was about to smile at him when I heard a huge “BANG!”  I stood there, stunned, and a second later realized I had a really bad pain in my side.  The teacher, who a second ago was smiling at me, came running at me.  And he looked very angry.  He started yelling at me, and I couldn’t understand what was going on.

I looked down while he’s angrily asking me questions, only to realize that he was angry because I was the source of the huge noise I’d just heard.  I saw one of the thermoses, which had formerly been under my arm, had blown off the bottom and was laying in pieces on the floor.  The pain I’d felt had been the thermos ejecting itself from my grasp.

Despite all my efforts to avoid being blown up by a pipe bomb, I’d managed to make a small explosive device anyway.  I’d forgotten the ideal gas law…and over the past couple hours, it appeared that pretty much all of the dry ice had sublimated.  A thermos full of solid dry ice turning to gas was going to create an extremely large amount of pressure.

I finally explained to the teacher that I wasn’t a student trying to blow up the school, despite the fact that I apparently looked young enough to be a student (at 28?!) and was standing in front of the administrative offices when the incident occurred.  I explained who I was and what was going on…and once he realized I was half in a state of shock, he started helping me to clean up my thermos pieces.

And people wonder why I don’t want to be an experimentalist.

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Comments»

1. Katiesci - November 27, 2011

Holy crap! Scary business! At kid’s events, we used to crush dry ice which the kids would scoop into a test tube, we’d put a balloon on the end of the tube and invert it to get the ice in, then tie it off. The kids would walk around shaking the balloons as they got bigger as the CO2 sublimated. The more daring ones would, of course, fill the tube and have their balloons explode. They loved it but balloons have no dangerous parts!

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