why is ice lighter than water?

Mike Mitchell m.mitchell at icrf.icnet.uk
Thu Jun 20 06:52:55 EST 1996


In article <Pine.SOL.3.92.960617152153.26044A-100000 at hsc.usc.edu>, "Chang
H. Choi" <cchoi at hsc.usc.edu> wrote:

> Hi, I am taking a biochem 101A now.  If any of you out there can explain
> why the density of ice is lighter than liquid water, it would be great
> help to me.
> I know the problem is related to the bond length of hydrogen bond, but I
> cannot fully understand the concept.
> help me.

When water is in the liquid state each molecule can freely move around each
other making and breaking hydrogen bonds. When water is in the solid state,
ie ice, a crystal structure is formed via the hydrogen bonds, however the
average inter-molecular distances is greater than in the liquide state. This
menas that there are fewer molecules in any given volume, and the density
is less.

As water cools the molecules come closer together and any individual h-bond
lasts longer, consequently the density increases. At 4 degrees centigrade,
however the h-bonds between the molecules start to form the crystal lattice
and the ice expands as the density increases.

-- 
Michael Mitchell                    "Smoke me a kipper,
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http://www.biu.icnet.uk/~mike



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