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Why didn't my triple point experiment work?

  1. Apr 16, 2008 #1
    So I'm trying to come up with some neat experiments to perform for some kids and I was thinking of showing them the effect of different temperature and pressure on the phase changes of water. Specifically, I was interested in subliming water by freezing it in a flask under reduced pressure and then having it go right into the gas phase after heating it up. The triple point diagram looks like this:

    http://www.chemguide.co.uk/physical/phaseeqia/pdh2o2.gif


    So I froze the water in dry ice down to about -70 C and then put the flask under vacuum. I let the flask warm up, but alas, it went back to the liquid phase rather than straight to the gas phase. What went wrong? Is it the vapor pressure of the water?
     
    Last edited: Apr 16, 2008
  2. jcsd
  3. Apr 16, 2008 #2
    You probably didn't have the flask under a hard enough vacuum. What were you using to pump it down?

    The triple point diagram you were going off of doesn't look very reliable, as it doesn't appear to be to scale, and has no markings on the axises.

    So, I looked up this one;

    http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/examples/941.jpg

    According to it, you'd have to have that flask down to a vacuum of around 0.01 atmospheres, which is pretty low. Did you have any way of knowing the vacuum in the flask?
     
  4. Apr 16, 2008 #3
    Yeah you are probably right about the vacuum. I just tried the experiment in the 10 minutes I had left before I went home. All I used was just the house line for the vacuum, which is probably not strong enough. I have no way of telling what the pressure is inside the flask, I will just have to go by what I see. Tomorrow I am going to try my high vac pump, which has a gauge on it. Attaining 0.01 atm should be easy.
     
  5. Apr 16, 2008 #4
    Yeah, definitely need a pressure gauge for this. Are you sure the flask can take that low a pressure? Don't need it imploding on you....
     
  6. Apr 16, 2008 #5
    Should be ok, we run them under high vac pumps all of the time with our products inside in order to dry them.
     
  7. Apr 17, 2008 #6

    chemisttree

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    Have you made any headway on the NO/CS2 demonstration?

    Have you thought about using the 'nylon rope trick'? Kids always love that one. I demonstrated that on career day for years.

    There was something I did that they liked even better... they liked it when I took my gloves off (inside out) and reversed them by quickly grabbing the opening and squeezing the partially air-filled glove. The fingers would 'pop' back into place. All that cool science was trumped by the popping of my nitrile gloves.

    I wouldn't spend too much time on the triple point experiment. It's not too interesting watching a bit of frozen water remain frozen in a flask while a noisy pump is running. Like watching paint dry.....
     
    Last edited: Apr 17, 2008
  8. Apr 17, 2008 #7

    chemisttree

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  9. Apr 18, 2008 #8

    Eh, the barking dog experiment is far too dangerous to do. My company doesn't want to be liable for anything. I decided to do some experiments with sulfur hexafluoride. Suck it in and you will sound like a demon when you talk. You can also play a "magic trick" on the kids by saying that you have "invisible" water. I will pour the SF6 into a container and float a boat on top of it, so it will look like the boat is floating on nothing. I will then have a kid from the audience come up and take a beaker full of the "invisible" water from the container and pour it onto the boat. The boat will then sink.


    I am also going to try to supercool some water. After shaking a container of super cooled water, you get a nice visual effect from the rapid growth of the ice crystal(s) like in this movie:

    http://video.google.com/videoplay?d...rcooled+water&ei=ki0JSN_HNJCUrgK1y_DiDg&hl=en

    http://video.google.com/videoplay?d...rcooled+water&ei=qy0JSIaCC4H4rQLezazzDg&hl=en
     
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