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Disappearing ice
 Q:
I noticed that when I leave ice cubes in the trays for a few weeks, they seem to get smaller. I always fill them to the
top, but sometimes the cubes have shrunk about 25%, so I don't think it's my imagination. Is this possible? Can
moisture evaporate from frozen water? -Ben R., Buffalo, New York
A: Yes, water can evaporate from ice. The phenomenon is called "sublimation". No, it's not your imagination. Ice cubes do
shrink in their trays, especially in frost-free refrigerators, which often are as dry as deserts (less than 20 % relative humidity).
[Corel] Ice cubes
The motion of the water molecules in ice cubes depends on the average temperature of the ice and is normally much less than
what it takes to break free of the ice surface. The kinetic energy (its energy of motion) of an individual molecule, however,
doesn't have to be the same as the average at a given instant and can be much greater. Consequently, a few high-energy
molecules do break free of the ice and enter the vapor state: they evaporate.
Frost-free refrigerators blow cold air into the refrigerator and freezer boxes, which keeps the air and the ice cubes evenly cool.
It also blows the water vapor from the ice cubes and prevents the vapor molecules from becoming ice again. That's why ice
cubes in trays can shrink as much as 25 % over a period.
(Answered by April Holladay, science correspondent, October 31, 2001)
Further Surfing:
USATODAY.com: Sublimation forms snow
USATODAY.com: What relative humidity means
The Saturday Scientists: Dry ice experiments
U of Florida: Frost-free refrigerators
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