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Density of Frozen Water

  1. Mar 19, 2014 #1
    When I freeze water in an ice cube tray in my freezer they expand slightly. Their weight remains the same therefore density has reduced. When I put these ice cubes in a glass of water they float because of the lower density. A small portion of the cube rises above the surface of the water.

    Is the volume of the cube which is below the surface equal to the volume of the original thawed water? And if so, why do people say melting antartic ice caps cause sea levels to rise? Wouldn't the ice simply melt and occupy the same volume as the frozen portion which is below the surface therefore sea levels would remain exactly the same?

    Cheers
     
  2. jcsd
  3. Mar 19, 2014 #2
    Yes, by Archimedes principle the weight of the volume of water displaced must be equal to the total weight of the floating object.

    As far as the melting ice caps go, you have to be aware of the two types of ice under consideration: sea ice and land ice. The former is floating in the arctic and antartic seas and varies considerably up and down over the year. The latter are the ice masses and glaciers that have built up over a long time on land. It is the melting of land ice that contributes to the rise in sea levels.

    And as a passing note, it is currently the arctic sea ice melting while the antartic sea ice has been actually increasing. However, the large majority of land ice on the planet has been steadily decreasing.
     
  4. Mar 19, 2014 #3
    Ok thanks
     
  5. Mar 20, 2014 #4
    Also note that thermal expansion of ocean waters may lead to sea level rise even if no ice melts.
     
  6. Mar 20, 2014 #5
    The volume of the cube which is below the surface is less than the volume of the original thawed water.
    The total volume of the cube which includes the ice above the surface is equal to the volume of the original thawed water.
     
  7. Mar 20, 2014 #6
    Doubtfull as water is at it's densest at 4 degrees so the oceans will contract before they expand if no ice melts.
     
  8. Mar 20, 2014 #7
    Unless I am misunderstanding you, this is not correct. Ice does indeed expand when it freezes, so the volume of an ice cube will be greater than the same cube when melted.
     
  9. Mar 20, 2014 #8
    This is just not correct. Water expands something around 10% in volume when it transitions to ice.

    Let V1 and V2 be the volumes of water and ice cube respectively.

    Let V3 be the portion of the ice cube submerged below the water line.

    By definition:

    V2 = 1.1*V1

    The weight of the ice cube is the same as the water before it froze = gamma*V1

    By Archimedes principle, the buoyant force acting on the ice cube = gamma *V3

    In order to be floating the buoyant force must be equal to the weight of the ice cube:

    gamma*V3 = gamma*V1

    which means that V3 = V1
    = V2 / 1.1

    Therefore, the submerged volume V3 is equal to the volume of water V1 and is less than the volume of the ice cube V2.
     
  10. Mar 20, 2014 #9
    Not correct. We are talking about sea water whose maximum density is close to its freezing point which in turn depends on the salinity and pressure.
     
  11. Mar 20, 2014 #10
    The ocean's temperature is mostly above 4 C. Any warming will lead to expansion, not contraction.

    Note that we're talking about temperature near the surface (where warming is currently happening, not about deep ocean waters which are indeed below 4 C but are not currently changing.
     
  12. Mar 21, 2014 #11
    Depends if the water talked about is liquid or solid.
    I probably misunderstood davies was the cube thawed or not.
     
  13. Mar 21, 2014 #12
    Yeah, it doesn't really depend on that if we are talking about seawater as the OP was doing. I think you probably misunderstood that he wasn't talking about pure water but rather was talking about sea ice and how its melting would affect sea levels (which it doesn't).
     
  14. Mar 21, 2014 #13
    Actualy I am not sure about the misunderstanding which the OP could clear up.
    What he said.
    If he had said water instaid of thawed water then there would be no misunderstanding on my part.
    By mentioning that the water was thawed could mean that he is talking about volume of the cube when it has melted and the amount of melted water above and below the surface.
     
  15. Mar 21, 2014 #14
    We are also talking about ice which when melts or does not is at it's densest at 4 degrees.
     
  16. Mar 21, 2014 #15
    Are we not also talking about the melting of ice and sea ice which is still at it's maximum density at around 4 degrees.
    The freezing point of sea ice is lower than freshwater ice due to salinity and pressure also has an effect.
    But salts in the ice can be excluded when it freezes so it becomes fresher won't this fresh water behave like normal ice when it thaws becoming denser at 4 degrees?
    http://www.britannica.com/EBchecked/topic/531121/seawater/301668/Density-of-seawater-and-pressure
     
  17. Mar 21, 2014 #16
    The Antarctic (and Greenland) have land ice, which will contribute to the sea level rise. This ice isn't floating on water.

    There's no land under the Arctic ice cap, where your argument is relevant.

    A major component of sea level rise will be thermal expansion of water. Incidentally, hot water is also an essential factor in the formation of hurricanes.

    One reason that the receding of the ice caps is relevant is that it's a measure of warming.

    Another reason is that snow and ice reflect heat much more than water, meaning that the lower the surface area that they cover, the more of the heat from the sun is absorbed by the earth, which contributes to warming, in a positive feedback loop. Positive feedback loops lead to instabilties.

    That's just the physics (and a little bit of geography) of it. Sadly Climate Change is a banned topic on these forums so we have to be careful, but I guess it's still ok to talk about the physics.
     
    Last edited: Mar 21, 2014
  18. Mar 21, 2014 #17
    There are two different issues:

    1) The OP was talking about sea ice which has salt in it. The OP just made analogy of an ice cube since the physics of buoyancy is the same for pure water and sea water but the density and melting points are not.

    2) You brought up the issue of the density of water being a maximum at 4 degrees and assumed that it was the same as ocean sea water, which it is not.
     
    Last edited: Mar 21, 2014
  19. Mar 21, 2014 #18
    Not true, again. Look at your own referenced link. It shows the maximum density of sea water with a salinity of 10 or more is actually at 0 degrees, not 4 degrees.

    When sea water freezes (around -2 degrees) the salinity remains in the ice, it does not fall out of the solution. But, even if it did, it would be irrelevant to the OP's question and it is also irrelevant to your false statement about the density of sea ice being at 4 degrees.
     
  20. Mar 21, 2014 #19
    Why is it banned? (I can guess the reasons but just want to know the official reasons for this censorship.)
     
  21. Mar 21, 2014 #20

    So even if it salt did fall out of the solution and became fresh water ice it would not become densest at 4 degrees when it thaws.
    How come? Fresh water is fresh water even if it had salt in it at any time.
     
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