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Water Freezing
Name: Shannon
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 1999-2001
Question:
The freezing point of water is 32 degrees Celsius. Is
this the temperature at which water freezes, or does water freeze below
32 degrees? The reason I ask is that I am an editing a children's text
book explaining the basics of weather. We have a statement in our book
that I believe may be inaccurate. Here is the statement: "Ice forms when
the temperature goes below 32 degrees." Is this correct, or does ice form
when water reaches 32 degrees? Which is it - at or below?
Replies:
Shannon:
A couple of points:
1. Ice MELTS at 32 degrees FAHRENHEIT not CELSIUS.
Don't confuse the kids on that. The point here is if I have a piece of ice
below 32 F it will be solid. If I heat the piece of ice slowly it turns into
liquid water at 32 F. The reason you see ice cubes floating around in warmer
water or other liquids is that the heat must be transported from the warmer
liquid to the ice for it to melt and in the real world this takes time.
2. Going in the other direction, that is, cooling liquid water or water
vapor, things are more complicated. Both liquid water and water vapor can be
cooled to temperatures lower than 32 F without immediately forming solid
ice. Sometimes in carefully controlled conditions for quite a long time. The
reason for this is for liquid water, or water vapor, to form ice crystals,
there must be a site for the formation of the ice to begin. When that does
ice formation is very rapid. These non-equilibrium conditions are said to be
supercooled.
So saying, "Ice MELTS at 32 F." is a far more certain statement than
saying, "Liquid water [or vapor] FREEZES at 32 F." because under some
conditions it doesn't happen immediately.
Vince Calder
Shannon -
Thanks for wanting to get the facts correct. Water freezes at 32 degrees
Fahrenheit and 0 degrees Celsius. (These are the same temperature on
different
scales.) Of course it will also freeze below that temperature. There are
times when the water does not have enough energy to move the molecules
into the
crystalline arrangement before the temperature of the water drops lower. This
can happen in a number of ways. One possibility are rain drops... called
super-cooled droplets. When these strike something such as the ground, a car,
or an airplane in flight, the energy from the strike provided the energy for
the water to instantly change into ice.
Also, the temperature is a measure of the average energy of a molecule This
means that some molecules in 32 degree water have more energy and are not
candidates for forming a crystal (freezing, that is). Others will be less
than
32 degrees Fahrenheit.
For your kids book... assuming younger kids... I think "water freezes at 32
degrees Fahrenheit" would work fine and not be misleading them.
Larry Krengel
The melting point of frozen water under normal atmospheric conditions is 32
degrees FAHRENHEIT (not Celsius). This is well established in most
texts. There is some problem with calling it the freezing
point. Microscopic ice crystals actually start forming below 4 degrees C
(about 40 degrees F), which accounts for the densest liquid water gets at 4
C. As more of the ice crystals form, the density of water decreases a
little bit until the water actually changes phases to ice. One would
normally think that this happens at 32 F, but you can get super cooled
water that remains a liquid until 29 F and then rapidly freezes. This is
one of the events that occurs when there is freezing rain. The water
droplets are actually super cooled, and freeze on contact.
Depending on the age level for the book, you may want to include all of
this. If it is for a young audience, then I would stick with ice melting
at 32 F, but water freezes at or below 32 F.
I hope my colleagues who also answer this question agree with me!
---Nathan A. Unterman
My understanding is that it freezes AT 32 degrees. If you look at a
chemistry or physics text book, look up melting point or boiling point. you
will find a graph of the states of water...according to this, it freezes at
32. good luck
Katie Page
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