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Homework Help: Latent heat and phase change question

  1. Sep 2, 2010 #1
    1. The problem statement, all variables and given/known data
    A 100g cube of ice at 0 degrees is dropped into 1.0kg of water that was originally at 80 degrees. What is the final temperature of the water after the ice has melted?

    2. Relevant equations

    Q = ml

    Q = mc(Tf - Ti)

    3. The attempt at a solution
    i'm not sure again about the temperature, but this is what i did:

    1) Qice = mc (Tf - Ti) where, m = .100kg, c = 2090, Tf = NOT SURE, 80 degrees??, Ti=0

    Q = ml where, m = .100kg, L = 3.33 x 10^5

    Qicewater = mc (Tf - Ti) where, m = .100, c = 4186, Tf = NOT SURE, Ti = 0

    Qwater = mc(Tf - Ti) where, m = 1.0, c = 4186, Tf = ?, Ti = ?

    can someone help me please?
  2. jcsd
  3. Sep 2, 2010 #2
    1st you need to find the latent heat of fusion of water
    then water: Tf=??? Ti=80
    ice: Ti=0 Tf=Tfwater=??
    then add up the energys
  4. Sep 2, 2010 #3
    I know i'm suppose to add up the energies, but I'm not sure about my temperatures.

    what's the final temperature of the ice? Initial temp is 0 degrees, but what's the final?? is it 80 degrees?
  5. Sep 2, 2010 #4

    Doc Al

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    Staff: Mentor

    If the final temperature was 80 degrees, you'd already have your answer!

    The trick is that all the water will end up at the same final temperature, which you have to solve for. That's your unknown.

    Set up your heat flow equation and solve for Tf.
  6. Sep 2, 2010 #5
    but if you look at my first posting...what's the initial temperatures for Qicewater and Qwater?

    I always get stumped on temperatures and also on phase changes. FOr example, if a block of ice is put into water, the phases are: ice, ice water, water...right?
  7. Sep 2, 2010 #6

    Doc Al

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    Staff: Mentor

    The initial temp for Qwater is given as 80 degrees. When the ice just melts, what is the temp of the water? (Does temperature change during a phase change?)

    I'd just call it ice and water. (But the water changes temperature as it gains/loses heat energy.)

    The mass of ice melts, then increases its temperature to the final temperature.

    The mass of water loses heat and thus lowers its temperature to the final temperature.
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