Editing Flash freezing (section)

== Impact of freezing ==

The surface environment does not play a decisive role in the formation of [[ice]] and [[snow]].<ref name=":4">{{Cite web |title=Freezing water droplets form sharp ice peaks |url=https://www.sciencedaily.com/releases/2012/10/121005092911.htm |access-date=2017-01-17 |website=sciencedaily.com}}</ref> Density fluctuations within water droplets result in the possible freezing regions covering both the interior and the surface<ref name=":1">{{Cite web |title=How water droplets freeze: The physics of ice and snow |url=https://www.sciencedaily.com/releases/2016/06/160621115439.htm |access-date=2017-01-17 |website=sciencedaily.com}}</ref>—that is, whether freezing from the surface or from within may be at random.<ref name=":1" />

There are phenomena like [[supercooling]], in which the water is cooled below its [[freezing point]] but remains liquid if there are too few defects to seed [[crystallization]]. One can therefore observe a delay until the water adjusts to the new, below-freezing temperature.<ref name=":3">{{Cite web |title=Superradiant matter: A new paradigm to explore dynamic phase transitions |url=https://www.sciencedaily.com/releases/2015/03/150318101355.htm |access-date=2017-01-17 |website=sciencedaily.com}}</ref> Supercooled liquid water must become ice at {{Convert|-48|C|F}}, not just because of the extreme cold, but because the [[molecular structure]] of water changes physically to form [[tetrahedron]] shapes, with each water molecule loosely bonded to four others.<ref name=":0">{{Cite web |title=Supercool: Water doesn't have to freeze until -48 C (-55 F) |url=https://www.sciencedaily.com/releases/2011/11/111123133123.htm |access-date=2017-01-17 |website=sciencedaily.com}}</ref> This suggests the structural change from liquid to "intermediate ice".<ref name=":0" /> The crystallization of ice from supercooled water is generally initiated by a process called [[nucleation]]. The speed and size of nucleation occurs within [[nanosecond]]s and [[Nanometre|nanometers]].<ref name=":2" />

As water freezes, tiny amounts of liquid water are theoretically still present, even as temperatures go below {{convert|-48|C|}} and almost all the water has turned solid, either into crystalline ice or amorphous water. However, this remaining liquid water crystallizes too fast for its properties to be detected or measured.<ref name=":0" /> The freezing speed directly influences the nucleation process and ice crystal size. A supercooled liquid will stay in a liquid state below the normal freezing point when it has little opportunity for nucleation—that is, if it is pure enough and is in a smooth-enough container. Once agitated it will rapidly become a solid.

During the final stage of freezing, an ice drop develops a pointy tip, which is not observed for most other liquids, and arises because water expands as it freezes.<ref name=":4" /> Once the liquid is completely frozen, the sharp tip of the drop attracts [[water vapor]] in the air, much like a sharp metal [[lightning rod]] attracts [[Electric charge|electrical charges]].<ref name=":4" /> The water vapor collects on the tip and a tree of small ice crystals starts to grow.<ref name=":4" /> An opposite effect has been shown to preferentially extract water molecules from the sharp edge of potato wedges in the oven.<ref name=":4" />

If a [[Microscopic scale|microscopic]] droplet of water is cooled very fast, it forms a [[glass]]—a low-density [[amorphous ice]] in which all the tetrahedral water molecules are not aligned but amorphous.<ref name=":0" /> The change in the structure of water controls the rate at which ice forms.<ref name=":0" /> Depending on its temperature and pressure, water ice has 16 different [[crystalline form]]s in which water molecules cling to each other with [[hydrogen bond]]s.<ref name=":0" />