How do ice crystals chemically form?

[cameron marical]

How do ice crystals chemically form? I was curious, i know that water expands when frozen, but what makes it actually expand?

 

Thanks.

[Mr Skeptic]

Ice expands because of the shape of the molecule and the crystal structure. Basically, when the molecules align as required to crystallize, there are a few "holes" which might be better filled if the molecules weren't aligned.

 

Under pressure, a form of ice can form that is more dense than water.

[cameron marical]

How does a lower in temperature make molecules align differently?

 

Thanks.

[hermanntrude]

A water molecule can be involved in up to four hydrogen bonds (google it if you dont know wat a hydrogen bond is)

 

In liquid water, each molecule is involved in perhaps 2-3 hydrogen bonds, and they are constantly forming and breaking to allow movement throughout the liquid. In solid water (ice), each molecule is involved in all four of its potential hydrogen bonds, which makes the whole structure stuck in a rigid tetrahedral arrangement, with big spaces in it.

[UC]

Well, temperature is the macroscopic realization of the kinetic energy of molecules.

 

I'm fairly sure that the answer you want involves the sharp and well-defined point where semi-free motion of the water molecules (water) is a lower energy state than vibrating rigidly in position (ice), but I don't know enough about this topic to say much. Ask a physicial chemist.

[hermanntrude]

here's a slide from my lectures which might help:

 

(attached)

 

That will be $59.99

[Theophrastus]

Yeah, pretty much elaborating upon what others have said, a decrease in temperature, while not always resulting in a greater or lesser density, will result in an increase or decrease in entropy. A lesser entropy, then allows for less interference with intermolecular forces (hydrogen bonding, in this case), and as such more water molecules bond themselves more efficiently, however, due to the rather interesting structure, of the molecular lattice, the space, which the molecules take up is greater. (I know, a rather crude explanation...)

[UC]

will result in an increase or decrease in entropy. A lesser entropy, then allows for less interference with intermolecular forces (hydrogen bonding, in this case), and as such more water molecules bond themselves more efficiently

 

Whhhhhhattttt?!?!?!?!?!

 

Also, methinks you're trying to explain this bass ackwards. The fact that the water molecules are ordered *is* lowered local entropy. It's not cause/effect. This is like saying that a lower temperature causes less vibration in the molecules. This is nonsense, since the vibration is the temperature. That heat had to go somewhere, so entropy overall will always increase (or stay the same).

Edited April 27, 2009 by UC

[Theophrastus]

(Theophrastus shrinks back slightly)

Yes, I know, bad use of words, as entropy is not an external topic, but as intermolecular forces, temperature, are all based on kinetic energy, so I tried to break it up, but yeah, in a rather inept manner...