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Thermodynamics phase changes steam and water

  1. Feb 26, 2012 #1
    1. The problem statement, all variables and given/known data

    In each case, the following are mixed in a perfect calorimeter with no heat lost or gained from the system. Find the final temp in each case.

    12.5 g of steam at 100C with 344 g of water at 22.2C.

    2. Relevant equations

    Q=Mc(Tfinal-Tinitial)
    M is the mass in kg
    C is the specific heat given by whatever substance we are dealing with
    And the parantheses is the change in temperature

    3. The attempt at a solution
    So I didn't really know where to start with this problem I realize that one of the substances is going to undergo a phase change but am unsure how to find which one does it. If I could help with that I believe I could calculate the final temperature.
     
  2. jcsd
  3. Feb 26, 2012 #2

    PeterO

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    Steam exists at 100o or higher.
    Water exists at 100o or lower. [but not below 0o]

    Given that one quantity starts at 100o and the other starts at 22.2o, it is reasonable to assume that the final temperature will be somewhere in between those two - so which one do you now think will change phase.
     
  4. Feb 26, 2012 #3
    The steam should become water since water boils at 100C. So it should just be common sense? Or is there a calculation I should be making?
     
  5. Feb 26, 2012 #4
    So since the steam is undergoing a phase change to become water. I'm not sure what to multiply the mass for since it isn't fusion or vaporization.
     
  6. Feb 26, 2012 #5

    PeterO

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    Have you heard of melting and condensation?
     
  7. Feb 26, 2012 #6
    Yes my physics book doesn't give those values though so I hadn't even thought I could use those.
     
  8. Feb 26, 2012 #7

    PeterO

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    Your book does give that - you just didn't recognise it.

    The energy input need to vaporise some water equals the energy returned when that (now) vapour condenses.

    Same thing with fusion and melting.
     
  9. Feb 26, 2012 #8
    So it's just the sign that changes
    . Thanks very much
     
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