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Does ice melt linearly?

  1. Aug 27, 2012 #1
    I'm trying to find possible errors in this lab experiment that I did, and one question I am thinking about is "Does ice melt linearly?" I assumed that it does so in my calculations, but now I'm not so sure.

    I measured the mass of melted ice (water) over a period of time and assumed that it melted at a constant rate. Since the density of ice is intrinsic (ρ = m/V) and would be constant, it made sense that the volume would change at the same rate as the mass. So that led me to believe that volume decreased at a constant rate too. Were these valid assumptions to make?

    Does ice melt linearly or by some other function of time?
     
  2. jcsd
  3. Aug 27, 2012 #2

    K^2

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    Short answer is no, not in general. If entire block of ice is at precisely zero degrees celcius, the rate at which it melts will be proportional to the rate at which heat flows into ice. That flow rate will depend on environment, surface area, shape, etc. If the core of the ice block is bellow zero, things get even more complex.
     
  4. Aug 27, 2012 #3
    I know about cooling a hot object ,
    Its something like of exponential curve ,
    as the room temperature nears , the graphs slope becomes smaller and smaller .
    Source: newtons law of cooling
     
  5. Aug 27, 2012 #4
    Ice does not melt linearly. If you research about Newton's Law of Cooling you will find that the Temperature profile will be expressed below:

    T(t) = Ta + (To - Ta)*e^(-kt)

    where
    Ta= ambient temperature
    To= initial temperature
    k= thermal conductivity
    t= time

    So the e expression makes it non-linear. Of course this is a very general version of the equation but it should help answer your question.

    Hope this helped
     
  6. Aug 27, 2012 #5

    russ_watters

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    Newton's law of cooling does not apply here since the ice is mostly at a constant temperature. K^2 had the answer.
     
  7. Aug 27, 2012 #6
    Well the point is that it doesn't decrease linearly whether the change in temperature is large or small, the change is non-linear.
     
  8. Aug 27, 2012 #7
    I guess , I wrote about cooling of hot object
     
  9. Aug 27, 2012 #8
    And melting of ice at constant temperature (0 deg) is an equilibrium state which shifts depending on the energy provided only
     
  10. Aug 27, 2012 #9

    russ_watters

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    No, it isn't/doesn't. [Pure] Ice melts at 0C, period. Increasing or decreasing the heat flux does not change that, it only changes the speed at which it melts. That's why you can use ice water as a good calibration point for a thermometer.
     
  11. Aug 27, 2012 #10

    rcgldr

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    Wouldn't the fact that surface area to volume (or mass) ratio increases as the size of the ice cube diminishes also be a factor?
     
  12. Aug 27, 2012 #11

    russ_watters

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    Yes, that's contained in what K^2 said, just without explaining the "surface area" part....
     
  13. Aug 28, 2012 #12
    russ ,
    you mean to say giving or taking heat doesnt shift the water and ice equilibrium .
     
  14. Aug 28, 2012 #13

    russ_watters

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    Yes: Ice melts at 0C. There really isn't any "equilibrium".
     
  15. Aug 28, 2012 #14

    K^2

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    That is a type of dynamic equilibrium. It's not really a static picture. With no heat flow, at 0°C you'll have simultaneous melting and fusing that balance each other out. Adding or removing heat shifts the equilibrium between phases.
     
  16. Aug 28, 2012 #15

    Borek

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    Perhaps we are using different definitions of equilibrium, then. For me an isolated mixture of ice and water at 0 deg C is a perfect example of an equilibrium - neither amount of ice nor water changes.

    Actually it is an example of equilibrium given at the Newton site here. What I don't like about their explanation is that they claim there is no mass transfer between solid and liquid phase - as far as I can tell there is a mass transfer, it just goes in both directions with the same speed, so the masses of both phases don't change. Or at least that's what kinetic approach to equilibrium suggests.

    Edit: K^2 posted while I was composing my post and looking for examples, and stated more or less the same.
     
  17. Aug 28, 2012 #16
    please explain me .
    Do ice and water exist in dynamic equilibrium ( like we study in chemistry equilibrium of ions, le chatelier principle stuff ) ?
    And if heat is provided equilibrium shifts towards water side , ie more of water molecules ?
     
  18. Aug 28, 2012 #17

    russ_watters

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    I thik we might be arguing different things. I'm just talking about temperature: melting temperature is not an equilibrium, it is a constant.

    Melting rate vs heat flux is an equilibrium.

    Consider the other side of the coin: your house's air conditioning. If you turn it on and let it run, various competing influrnces will fight each other and eventually your house will reach an equilibrium temperature.
     
  19. Aug 28, 2012 #18
    ok , umm
    I am talking about concentration of water and ice molecules coexisting in ice water mixture , and their change in concentration when heat is provided(shifting of equilibrium) . Till no ice remains .
     
  20. Aug 28, 2012 #19

    russ_watters

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    Agreed: In a well insulated container, concentration is an equilibrium. That's pretty different from what the OP was asking though.
     
  21. Aug 28, 2012 #20
    I guess he asked about melting of ice .
    So it is nothing but shifting of equilibrium towards more favorable product ie water .
     
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