How Everything Works
How Everything Works How Everything Works

Question 1173

What happens to ice when it is left in the freezer—does it evaporate? I have noticed that over time the ice cubes shrink? — J & K
When you leave ice in a frostless refrigerator, it gradually sublimes and shrinks away to nothing. Sublimation is equivalent to evaporation, except it involves a solid converting directly into a gas. The surface of an ice cube is a busy place, with water molecules landing and taking off all the time. If more water molecules land than leave, the ice cube will grow in size. If more water molecules leave than land, the ice cube will shrink. The water molecule landing rate is determined by how much moisture there is in the air. In a frostless refrigerator, the air is extremely dry, meaning that it contains very few water molecules. Thus the landing rate in a frostless refrigerator is very low and the ice cubes shrink. If you watch the ice cubes in an older style refrigerator, you will find that they grow over time because the air in that refrigerator is moist and the landing rate is high. Incidentally, this sublimation of water molecules from ice is why snow disappears gradually even when the weather remains cold and is also how freeze drying of food is done.

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