| MadSci Network: Chemistry |
An interesting question, which relates the peculiar shape & properties of the water molecule.
In both liquids and solids, the particles are about as close together as their movement will allow. This is why both liquids and solids are incompressible. Generally, when you heat up a liquid or solid, its particles, which are now moving slightly faster, will jostle each other a little farther apart. On the scale we can see, this results in expansion.
In a gas, the kinetic (motion) energy of the particles overcomes the forces which attract particles to each other, so the particles actually spread out a great deal. Because of the space between gas particles, a gas can be compressed.
What makes water act a little differently? First, there is its shape. The water molecule consists of two fairly small Hydrogen atoms, connected about 120 degrees apart to a larger Oxygen atom. So the water molecule looks sort of like a microscopic boomerang with a really fat middle. Elbow macaroni would make a reasonable model, for our purposes.
. . (Oxygen - slightly negative; likes electrons)
O
/ \
H H (Hydrogens - slightly positive; e- drawn to O)
The next important factor is Polarity. The water molecule is polar, which
means it has areas of positive and negative charge (sort of like static
cling). The oxygen atom tends to hog electrons, so it is slightly negative
and the hydrogens are slightly positive. Consequently, the hydrogens of
one water molecule tend to stick to the (oppositely charged) oxygen of the
next.
At temperatures where water is a liquid, this has little effect on how the molecules arrange themselves. Think of taking a bowlfull of those macaroni and shaking them around - they tend to pack in pretty well.
However, when water begins to freeze, the effects of polarity are considerable. Hydrogens link up with neigboring oxygen, to form an orderly crystal structure. Generally, this structure is hexagonal, which is why snowflakes have six sides. Think of taking all that macaroni and gluing it together, so that the end of one noodle attaches to the middle of the next. Of course, I don't recommend actually _doing_ this unless you have a really bored Cub Scout troop on your hands.
Once you have glued all of your (hopefully, imaginary) noodles together, you'll notice that they aren't packed as closely together as when they were loose. Water is they same way, which is why it expands, about 10% on average (which, incidentally, is why we see the top tenth of an iceberg).
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