We did not have much snow where I was on vacation but there were really nice ice crystals.
I guess the crystals were formed like rime. Deposition of water vapour at night when the equilibrium pressure of water vapour drops because the temperature is lower. On the next image you see one of the crystals on my finger. You notice the lines that are probably just like growth rings, indicating the ice that was added every day.
Ice Ih which is stable at ambient conditions crystalises in the hexagonal system. This can be seen here.
The most notable property of ice is that its density is lower than that of cold water. This is caused by the fact that empty spaces remain when the molecules are arranged to form ideal hydrogen bonds. The fact that ice is losely packed makes it plausible that its structure will change when pressure is applied to it. That is why there are so many high pressure modifications of ice. You can find the phase diagram and much more information here.
The structure stable at ambient conditions is called Ih. It crystalises in a hexagonal system. If you cool it down to about 170 K you change from hexagonal to cubic, the structure is called Ic. An important property of those two structures is proton disorder. Each oxygen has to have two short covalent bonds and two long H-bonds. In ice I modifications hydrogen atoms can be randomly arranged. This results in a fairly large remaining entropy at absolute zero (3.5 J mol-1 K-1).
Below 70 K ice XI is stable. It is like Ih only that all hydrogens are ordered. At low temperatures entropy is not important any more and a slight difference in enthalpy might come into play.
Ice II is formed at 198 K and 300 MPa. It may be the main component of icy moons. It has no proton disorder. Because of that you notice an entropy change of -3.22 J mol-1 K-1 when transforming ice Ih into ice II.
Ice III and ice IX are corresponding structures where ice III has proton disorder and ice IX doesn't. Ice IX is stable at much lower temperatures. The reason is the same as with ice I and ice XI.
Ice VII is stable above 3 GPa it is in equilibrium with supercritical water. Ice X at 100 GPa does not differentiate between covalent and hydrogen bonds.
I picked out what sounded interesting to me. For more information check out (http://www.lsbu.ac.uk/water/).
A Defense of Journal Impact Factors - Vilified, journal impact factor may still be useful for scientists. But use it with caution.
3 days ago