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Explanation of Ice Cube in water.

  1. Apr 29, 2012 #1

    Could someone please explain what happens when an ice cube is put into a glass of hot water? I'm talking about the phase change and how the water gets cold.

  2. jcsd
  3. Apr 29, 2012 #2
    when you put it in water there is an overwhelming probability that net energy from the ice cube will transfer to the water
  4. Apr 29, 2012 #3
    Could you expand? I mean is there a transfer of heat before, after, or during the phase change from solid to liquid?
  5. Apr 29, 2012 #4


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    You whaaaaat?
    Are you proposing to do something very extraordinary with the pressure? Doesn't thermal energy go from Hot to Cold in this Universe?
  6. Apr 29, 2012 #5
    To begin with, think of "heat" simply as kinetic energy being transferred around between molecules. The transfer happens when molecules collide: if you strike a slow moving ball with a faster ball, the slow one moves a bit faster and the fast one a bit slower.

    It's about the same with molecules and kinetic energy.

    When you put the ice cube in the water, it has a lower temperature and therefore less heat (transfer of "movement") per gram than liquid water (so ice is like the "slow ball.") The liquid water molecules, which are moving faster and have higher kinetic energy, give the molecules in the ice some of their kinetic energy when they collide.

    Remember that solid water is a crystal, and its molecules don't really move. The transfer of kinetic energy causes molecules to start moving, which disrupts the crystal and turns it into liquid water. So I suppose the transfer of heat occurs "during" the phase change.

    The water becomes cold since it loses (overall) kinetic energy to the ice, the same way the fast-moving ball loses momentum to the slower one.

    As sophiecentuar said, thermal energy goes from "hot" to "cold." Kinetic energy transfers indicate heat; cold is only the absence of these transfers. Curl is at least right that there is a net energy transfer.

    ...Mm. I'm not too good at explaining things, sorry. But I hope you find it a little bit helpful.
  7. Apr 29, 2012 #6


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    The heat moves from the water to the ice cube as soon as the ice cube touches it. It is constantly being transferred before, during, and even after the phase change until the temperature is at equilibrium.
  8. May 1, 2012 #7
    oops lol, the other way around.
  9. May 1, 2012 #8
    Consider the hot water and the ice cube to be the two components that together comprise a system. Why is it that a small amount of ice can cool hot water to what we would speak of as "cool" water? It's because the energy contained in the crystal structure of ice, per gram of ice, is much greater than the energy that is contained in even a gram of very hot water. The specific heat capacity of liquid water is 4.184 J/g/K so that even water at 373 K, just below its normal boiling point, cooled down to 273 K, its normal melting point, will comprise an approximately 420 J/g of heat that can be transferred.

    By contrast, the latent heat of fusion for ice is 334 J/g. Imagine if you had a ice and liquid water in 1:1 mass ratio. A quick calculation using principles of calorimetry will show that liquid water initially at 175 degrees Fahrenheit, which is hot enough that you will most surely die if immersed in a bathtub of water that hot, can be cooled down to just above the freezing point of water by that equal mass of ice initially present.

    You could literally see the thermometer drop from water that will kill you because it's too hot to water that will kill you because it's too cold. That's the power of ice. =)
  10. May 1, 2012 #9


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    jay has the right idea. It takes some heat input to the ice to cause it to turn to liquid water, which is called latent heat of fusion. This heat energy is effectively used to break the close bonds of ice, to turn them into much looser liquid water bonds. (Which requires energy). The heat for the process comes from the hot water which the ice was placed in.

    At this point you might think "Hey! the temperature of the water went down and the ice turned into liquid water without necessarily increasing in temperature, so doesn't that mean that entropy has decreased!?" But actually, the ice has lower entropy than liquid water, so the entropy of the ice increases when it changes phase.

    EDIT: I said latent heat of fusion, but I think that term is used for the heat coming out when liquid turns to solid. But you see what I mean when the process goes the other way, it requires a heat input.
  11. May 1, 2012 #10


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    See this thermal conductivity problem/experiment I did on this.


    Note: Actually this equation is for steady state but the actual melting time was very close
    to calculated.
    scroll to equations at bottom of page
    Last edited: May 1, 2012
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