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Homework Help: Can Experiment (Pressure)

  1. Nov 3, 2008 #1
    I have a question regarding the classic experiment with a [soda] can involving pressure. Here is a simplified version of the experiment, without going into too much detail:

    1. Heat some water up.
    2. Put the water in the can.
    3. Invert the can inside an ice bath.

    The result is that the can is crushed.

    I mostly understand why this happens, but there is one part that I don't get. Here is my explanation:

    The air inside the can is rapidly cooled when inserted into the ice bath. As a result, the decrease in temperature causes a decrease in pressure. The pressure on the outside of the can is greater than the pressure on the inside of the can. Since this is true, the air outside exerts a force crushing the can. What I don't get is how the pressure becomes equalized.

    Equations used to describe the air inside of the can

    Equation 1
    Before the water is heated and added to the can
    PV=nRT (normal T and P)

    Equation 2
    Once the can filled with heated water and inverted into the ice bath
    (0.5)PV=nR(0.5)T (drop in T causes the drop in P; lets just say T drops by a factor of 0.5, so then the P side must drop by the same factor)

    Equation 3
    This is the part I don't get. Once the pressure on the inside is equal to the pressure on the outside of the can, the crushing force exerted on the can no longer exists. So if the pressure initially had a factor of 1 before the experiment, then once the experiment is over and the pressures become equal, shouldn't it also have a factor of 1? How does the equation jump from Equation 2 to Equation 3. Doesn't something on the right side have to increase to compensate for the normalization of the pressure?

    If I've messed up somewhere in my thought process, let me know.
  2. jcsd
  3. Nov 3, 2008 #2


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    In addition to decreasing pressure, what other variable could you decrease to compensate for a decrease in temperature?
  4. Nov 3, 2008 #3
    The volume, but even if the factor of 0.5 is split between volume and pressure then something on the right side still has to increase. Right?

    Unless only the volume decreased, because then the value of volume would stay low after the experiment... hmm...
    Last edited: Nov 3, 2008
  5. Nov 3, 2008 #4


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    Yes, the volume decreases until the pressure is equal to the atmospheric pressure.
  6. Nov 3, 2008 #5
    I think I get it now. Thank you very much for all your help.
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