water crystalization

[katch78]

Why does water crystalize when frozen?  I know it has something to do with the molecular structure, but why the angular, straight formations?

[Flurfl]

Here's a pretty cool link on water and ice (pun intended ;-)

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

H2O is pretty complicated stuff. However if you look at the link for hexagonal ice under the Phase diagram heading you'll see the structure of all the ice that you'll seen in normal conditions. The interesting thing about natural ice is that is less dense than water. This is unusual and cause ice to float in water. normally when a compound becomes a solid it will be more dense not less. In the case of water it is the hydrogen bonding that causes it to be more dense. I believe most compounds will form crystals in their solid phase. It's when you get mixtures of compounds that they are more likely to form amorphous solids. Here's a bit on snowflakes.

 http://www.its.caltech.edu/%7Eatomic/snowcrystals/primer/primer.htm

[Calvesy]

You might think in the molecule H2O that the hydrogen ions (protons) would repel to opposite sides of the oxygen atom, but in fact a boomerang shape is formed. This is because electrons in the the two bond pairs are repelled by two lone pairs. When the water freezes, weak electrostatic forces between the hydrogen ions and these lone pairs in a neighbouring oxygen electron shell attract to tile the boomerang shape on a scale visible to us in snowflakes and puddles.