A: FROM MENTOR ROSE CLARK IN PA
It is not a chemical reaction, it is just the interaction between water
molecules. Water molecules interact through a process called hydrogen
bonding. Hydrogen bonding is a interaction between a H on one water
molecule with the oxygen of another water molecule. Since all water
molecules can hydrogen bond they sort of stick together in the liquid as
well as the solid phase. As the temperature decreases the water
molecules organize based on the hydrogen bonding (partial positive
charge interacting with a partial negative charge) to form a beautiful
crystalline structure, called a snowflake.
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A: FROM MENTOR JOAN LUSK IN RI
Since snowflakes clump together more when the temperature is higher -
those big,composite flakes near O degrees Celsius vs tiny ones at 5
below - perhaps a bit of liquid water on the surface, forming and
re-freezing, helps to stick them together? Alternatively, perhaps
the growth of the snow crystal from the water vapor leads to sharing
water molecules across the interface between separate crystals. In
either case, it is hydrogen bonds that are holding the structure
together: HOH...OH2 (sorry I can't used subscripts here; I'm trying
to show H bonded to one O but also interacting with the non-bonding
electrons on another O.) We don't usually think of the making and
breaking of hydrogen bonds as a true chemical reaction, because the
energy of hydrogen bonds is so much less than that of covalent bonds.
But the difference is one of degree rather than a fundamental one.
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A: FROM MENTOR MOLLY WILLIAMS IN MI
It's a physical reaction, not a chemical one. Ice is an unusual
material because you can make ice melt by applying pressure. If you
take a handful of snow in your hands and squeeze hard, the ice melts
at places where the local pressure is the highest. When you release
the pressure, the water re-freezes, and you get a solid snowball.
This works best when the temperature of the snow is close to
thawing. If the temperature is very cold, you can't apply enough
pressure with your hands to cause the local melting, and the snow
can't be compacted to a snowball. This behavior of ice is very
unusual, and it's related to the fact that ice floats on liquid
water. Almost all other materials freeze to a solid that sinks in
its own liquid. The plants and animals that live in a pond would be
unable to survive winter unless ice formed and remained on top of
the waterinstead of sinking to the bottom.