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Name:  Laurie
Status: other
Grade: other
Location:NY
Country: N/A
Date: 12/19/2005


Question:
Water is at a maximum density just above freezing and therefore ice is lighter than water. As ice cools further it naturally contracts increasing its density. Is there a temperature at which this very cold ice becomes denser than liquid water?


Replies:
Based on density of ice tabulated in the CRC Handbook of Chemistry and Physics the density of ice at its vapor pressure (essentially zero) at -180 C (93 kelvins) is 0.9340 g/cm^2. At 0 C (273 kelvins) the density is 0.9167 gm/cm^3. The thermal expansion coefficient at -180 C is -(1/V)*(dV/dT) = 30x10^-6 C^-1, where V is the molar volume and T is the temperature. The referenced source warns that the data are rather scattered. However, it is not likely that further cooling will increase the density sufficiently to make up the difference in density between 93 and 273 kelvins. The story of "ice" is very complex, interesting, and important.

Ice has about a dozen crystal forms and several amorphous forms (molecules randomly arranged rather than in a crystal lattice) as well -- this is probably why the density data become scattered at low temperatures -- the molecules are still "moving around" and some are not yet in an equilibrium position. Ice, and the gases trapped in ice taken from Antarctic ice cores are studied for the information they may provide about the composition of the Earth's atmosphere in past years, centuries and eons. A note of warning here. In the kiloyear time frame there are many processes, such as the selective migration of gas molecules in ice, that are not well understood, so it is not a "simple" matter of measuring the composition of a gas bubble trapped in ice and assuming that represents the composition of an ancient atmosphere. In addition, there is the interesting question of the existence of water on Mars and the Moon. I have collected a few web sites (below) that will open the door for you to the fascinating story of our most important molecule, water. You will find many more links to take the story as far as you want.

http://www.lsbu.ac.uk/water/amorph.html

http://www.lsbu.ac.uk/water/phase.html

http://www.iceclimb.com/science.html

http://www.uwgb.edu/dutchs/PETROLGY/Ice%20Structure.HTM

Vince Calder


Water still presents challenges to our understanding, note the very recent citation in the 08 Dec 2005 issue of "Nature" page 715. From there it is possible to link with a bibliographic web site containing almost 1000 citations regarding the physics and chemistry of water:

http://www.lsbu.ac.uk/water/ref10.html

Vince Calder



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