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Homework Help: Isobaric process experiment with plastic tube and hot water

  1. Oct 14, 2012 #1
    Hi! I have this experiment, that keeps getting me frustrated. The experiment is simple:
    We have hot water and a long plastic tube with one end closed. We simply measure how much water is in the tube and from that we get the water volume (water is in cylinder shape).
    But there is something I don't understand: when water is hotter then there is less water in tube (about 1cm long cylinder). When the water is less hot, then thee is more water (about 3cm long cylinder).
    I can figure that the air which is at the end of tube is giving too much pressure so the volume doesn't add up.

    For example, if this is really a isobaric process, then pV = RTm/M (p = RTm/(VM) should be the same all the time, but the numbers aren't the same.

    If we want to threat water as an ideal gas, then the mass shouldn't change (could simply say that m/M is 1 mol), but the mass changes, because if the water expands because of heat, then there should be more water volume, not less. So the water actually flows away, can't really threat it as an ideal gas.

    What I'm missing here?
    It is possible that experiment is supposed to fail, but then I need a good reason. I don't think the mass change reason is a good enough reason.

    For example:
    length of cylinder - 0.008m
    Diameter of tube - 0.0037m
    Temperature - 337 K
    Volume - 0.86 * 10^-7 m^3
    Mass - 1000*0.86 * 10^-7 = 0.86*10^-4 kg
    Molmass - 1 * 10^-3 kg/mol
    Pressure - p = R*T*m/(V*M) = 8.31*337*0.86*10^-4/(0.86*10^-7*10^-3) = 2.8 * 10^9 Pa
    So basically I get that there is 10 000 atm pressure... very possible...
    Last edited: Oct 14, 2012
  2. jcsd
  3. Oct 14, 2012 #2
    Let me make sure I'm understanding this. You're heating a cylinder of water with the top end open. You take a measure of the height of the water and then let it cool. You take another measure and the water level is higher?
  4. Oct 14, 2012 #3
    I put a long plastic tube in hot water container. The water flows a little bit inside the tube, but not much. But when the water cools down, there is more volume of water inside the small plastic tube.

    I was thinking that I did something wrong, but everyone gets the same results.

    I know that density of water isn't 1000 at all temperatures, but it changes from 900 to 1000. That doesn't change much, we could say that this is what is making small number changes.
  5. Oct 14, 2012 #4
    I'm still having trouble understanding the setup. Could you draw a diagram or two?
  6. Oct 14, 2012 #5
    http://s18.postimage.org/bj0mv9kfb/exper.png [Broken]

    Sorry, should have drawn it earlier.
    That small water volume changes upwards when temperature drops.
    Guess we can assume that there is air at the closed end and the air pressure pushes out water.

    I guess the way it works is like this: Since the temperature rises, the air expands more and pushes out water. Pressure shouldn't change (theoretically) for the water, only the volume changes. But still, having trouble with those extra 0. I can change the length how I want, it stays the same with mass and each other crosses out. Only when I changed molmass to 1 kg/mol from 0.001 kg/mol then I get 28 atm. pressure, but still not right.

    But p=RTm/(VM) can't be used here. m = Density*V, so p = RT*Density*V/(V*M), the V cross out, so basically pressure is only depended on temperature.

    I understood now.
    I was missing the length of tube. I'm not supposed to look at water pressure, but at air pressure.
    The water in tube works as a cork, but changes in order to maintain pressure.
    Last edited by a moderator: May 6, 2017
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