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Frost-free Freeze Physics

"Sublimation" is when ice turns into a gas without passing through the usual intermediary liquid state. If you leave your ice cubes in back of the freezer for a month or so, you might notice they start to shrink. That's because they are sublimating: the ice is turning to water vapor even though it never melted.

But I tried this experiment myself and my ice cubes never shrunk! All I got was a cold head! Then, physicist John Carini explained to me that it only works if you have a frost-free freezer.

It's in the nature of sublimation. When we say that ice can turn into water vapor, what we mean is that fast-moving water molecules can actually fling themselves off the surface of the ice cube and into the surrounding air. That's what evaporation means. The more molecules that can free themselves of the ice this way, the more the ice has turned into vapor.

In order to do that, the air around the ice cube can't be saturated with water already. If it is, there's no room for more molecules and evaporation can't take place. Frost-free freezers keep the air from becoming saturated so that it won't freeze on the walls. But that also means ice-cubes can sublimate nonstop!

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