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Gibb's phase rule degrees of freedom

  1. Feb 3, 2015 #1


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    I am watching this video to try and better understand Gibb's Phase rule

    The part of interest starts at 4:30

    I like that he is using a visual to explain, which is very helpful. When there is 1 phase and one component, there are 2 degrees of freedom. This means two variables can be changed. This is clear when he shows the gas phase of the P-T diagram where changing either the temperature or pressure remains in the same region as a gas.

    He also shows when there are 3 phases, which is at the triple point, you have zero degrees of freedom. He shows the triple point on the P-T diagram, and if you change the either the temperature or pressure, you will no longer be at the triple point, hence will not have 3 phases.

    What seems to break down for me is when there are 2 phases. Thus there is one degree of freedom. He shows the line between the liquid and gas phase, and seems to justify that you can change either P or T, but not both, and you will still have two phases. However, when I look at it, it seems like if you change either P or T, then you will leave that gas/liquid phase line. It seems like you would need to change P and T precisely to remain on the line, thus having 2 degrees of freedom.
  2. jcsd
  3. Feb 3, 2015 #2

    Quantum Defect

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    You can also think about degrees of freedom as telling you something about the "space" that can be occupied on a P-T phase diagram.

    DoF = 2 ==> two dimensions. You are in the area between bounding lines in the T,P phase diagram. You can have lots of different T's and P's (you can vary both T and P at the same time, and still remain in the one-phase region)
    DoF = 0 ==> zero dimensions. You are at a single point. The triple point. There is only one T and one P that will work. Zero Degrees of freedom (you can't vary T, or P)
    DoF = 1 ==> one dimesnion. You are on one of the lines. A coexistence curve. For any T (there are many that are possible), there is only one P. One degree of freedom. (You can vary T, but then you are stuck with one P for any given T. Conversely, you can vary P, but then you are stuck with a single T for any given P.)
  4. Feb 3, 2015 #3


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    Ok that makes sense, that was what I was starting to think after I wrote my OP.
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