I am an engineering researcher at the
University of California-Santa Barbara. Thank you
for your GREAT question “Why it is that water is
less dense in solid form than in liquid form?”
This is a personal favorite question of mine, as
it has to do with a lot of the research that I do.
Typically, solids are always more dense than
liquids. So it is very interesting that water—the
most important molecule on our planet—is denser as
a liquid than it is as a solid. This occurs
because of the type of interactions that water
molecules have with each other. When two water
molecules come very close together, they can form
a special type of bond called a “hydrogen bond”.
The hydrogen bond forms between the hydrogen atom
of one water molecule and the oxygen atom of the
other molecule. These hydrogen bonds are very weak
(much weaker than chemical covalent bonds) but
they are still important. In the solid ice form,
each water molecule will form a hydrogen bond with
4 other water molecules. The 4 hydrogen bonds in
ice create a very ordered network of ice
molecules. However, when water is in its liquid
form, each water molecule can only form a hydrogen
bond with 3 other water molecules. Since each
water molecule only forms 3 hydrogen bonds, this
makes the arrangement of the water molecules a lot
crazier and disorganized. The overall result is
that, with 3 hydrogen bonds per molecule, the
water molecules are able to squeeze together more
tightly and create a denser liquid. I have
attached a link to an image below which will help
illustrate this process.
here to see image
The red balls represent the oxygen atom in water,
the white balls represent the hydrogen atoms in
water, and the dashed lines represent the hydrogen
bonds between water molecules. In the first
picture (a) you can see how solid ice forms 4
hydrogen bonds and is ordered very nicely. In the
second picture, you can see how liquid water has
fewer hydrogen bonds and the water molecules are
no longer ordered in a nice way.
I hope this helps with you question!
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