Expansion of water when it freezes
Name: bonnie j light
Why does water expand when it freezes?
Water expands when it freezes because of "hydrogen bonding."
This means that the hydrogen on an H2O has a strong attraction
for the "lone-pair", unbonded electrons on other nearby H2O
molecules. In crystalline ice, each oxygen atom is surrounded
by 4 hydrogen atoms (2 of its own and two from two other,
neighboring water molecules in the crystalline lattice). This
forms a "network structure" which is, incidentally, the same
as diamond's (but with weaker bonds). The network structure has
a lot of space between molecules. In fact, there is more
space between molecules in this network structure than there
is (on the average) in the liquid structure. Since there
is more space between molecules in ice than in liquid water,
ice is less dense.
Hope this helps,
Perhaps it will amuse you to know that there are at least eight
different forms of ice, each stable at different pressures and
temperatures. Ice VII, for example, is only stable ABOVE 0 deg C,
but also only at enormous pressures.
Only one form of ice (ice I) is less dense than liquid water.
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Update: June 2012