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Homework Help: Question about precipitation

  1. Oct 12, 2014 #1
    The question is in the attachment... "this is an old past exam question"

    for i) I assume that the pressure will remain constant (I'm not sure) since volume is likely to increase proportion to the decrease in temperature. (Is this correct?)

    Best Regards

    Attached Files:

  2. jcsd
  3. Oct 12, 2014 #2

    Simon Bridge

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    What leads you to believe that the volume increases when temperature decreases?
    Do you know an equation that relates volume to temperature for a gas?

    The graph shows how the saturation vapor pressure varies with temperature for "a parcel of air".

    You notice that the initial state vapor pressure is lower than the saturation vapor pressure - is it possible for the vapor pressure to be higher than the saturation pressure? i.e. what does "saturation vapor pressure" mean?
  4. Oct 12, 2014 #3
    Since it is a parcel of air, the volume is constant? thus the only that that will change is pressure? Can I use the ideal gas law in this case? "PV=nRT"

    I think it is nott possible for vapour pressure to be higher than saturated vapour pressure since it will precipitate (I'm not certain)
  5. Oct 12, 2014 #4

    Simon Bridge

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    Well you just said that the volume increases as temperature decreases - now you are saying the volume stays the same?

    Lets pretend that you can use the ideal gas equation - what does that equation tell you happens to the pressure (at constant volume) as the temperature decreases or the volume (at constant pressure) for that matter?
    Compare with what you wrote in post #1.

    However - the question is talking about vapor pressure - is this different from the gas pressure?
    Do you know any definitions for vapor pressure?

    ... then this is something you need to check. It looks like you need to revise your work on vapor pressure.

    Looking at the graph, if the temperature dropped, but the vapor pressure stayed the same, would the vapor pressure be above or below the saturation pressure?

    You may find useful:
  6. Oct 15, 2014 #5
    thank you for your help Simon, I have been going through much of my lecture note and recording in the recent days. It seems that the ideal gas law has nothing to do with this case. by using this equation saturation vapour pressure = 0.6108exp(17.27*T/T+237.
    3), I got saturation vapour pressure to be equal to 1.23 kPa. (This is correct right?)

    It seems like this poinrt (10, 1.23) is above the blue line, thus precipitation will occur

  7. Oct 15, 2014 #6

    Simon Bridge

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    That's better ;)
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