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Homework Help: Thermo physics (Phase diagram Phase changes)

  1. Oct 10, 2014 #1
    Hi have some thermal physics question would like to clarify. Thanks :)

    Question 1: For a substance in a pure phase heated at constant pressure, the heat absorbed is related to ____________
    (a)the molar heat capacity of the substance
    (b)the amount of substance
    (c)the change in the kinetic energy of the molecules
    (d)the change in the potential energy of the molecules
    (e)the average distance between molecules
    (f)the molar enthalpy of the phase change
    (g)the change in temperature

    Which option should be right?
    I have no idea for this question but in this question there should be no phase change right compared to the bottom question?? :
    (a) This should be constant? since there is no phase change?
    (b) remain the same ? (no phase change)
    (c) should increase since heat absorbed and Temperature increases so K.E increase
    (d) Not sure?? should increase also?
    (e) should not change? since the substance is at the same state (no phase change)?
    (f) Not sure
    (g) Should change. Since this is not a phase transition??

    So my answer is (c) (d) (g) . Is it correct? and did I miss out anything?

    Question 2: During a phase change of a pure substance, the heat transfer is related to _______________
    (a)the change in the kinetic energy of the molecules
    (b)the amount of substance
    (c)the molar heat capacity of the substance
    (d)the average distance between molecules
    (e)the change in the potential energy of the molecules
    (f)the change in temperature
    (g)the molar enthalpy of the phase change

    (a) Wrong since phase transition K.E does not increase.
    (f) No change in temperature at phase transition
    (g) Not sure?

    Is (b) (d) (e) correct for this? Any other additional option I missed up.
    Is (c) also correct? Since after a phase change for example solid to liquid phase, the heat capacity of solid and liquid also differs??
  2. jcsd
  3. Oct 10, 2014 #2


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    Staff: Mentor

    Hi tvtokyo. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    In Q1, I'm wondering why you dismissed (a) and (b)?

    Have you googled "molar heat capacity"? You can't hope to properly address the question if you don't know what the terms mean.

    The greater the mass that is heated through Δt, the greater the kJ needed, surely?

    Good luck with your studies.
    Last edited by a moderator: May 7, 2017
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