Does ice evaporate?

04 May 2008

Question

I have ice cube trays in my freezer as I’m sure most people do. I don’t use them all up that often. When I come to get them out over a few months time it seems to me that the level of the ice in the ice cube trays is much lower than when I filled them up with water a month or two before. Is the ice evaporating?

Answer

This is a really good observation and it's absolutely true. Your ice cubes will lose volume in your freezer over time. The reason is, although they're frozen, literally the water molecules have been joined together to form ice, what actually is going on (if you could zoom in with a very powerful microscope and watch the energy in ice) is that the ice is sharing out all the energy -even though there's a lot less of it because it's cold - amongst all the water molecules. And it's random. Every so often there'll be some water molecules that have enough energy to vibrate or escape free from the surface of the ice. At the same time others will rejoin onto the ice and this is what's called a dynamic equilibrium. Every so often you'll get a molecule of water which will gain enough energy to spring off of an ice cube and it might well form some ice elsewhere in the freezer or just fall out of the door. Over time we've got a lot of something, water as ice, it will slowly diminish and shrink. It will slowly disappear. It will deposit round the inside of the freezer but because the freezer has the door open from time to time you're going to be loosing water as well that way. Some water goes in from time to time if you open the door and you're got a stuffy room. Basically it's because there's a dynamic equilibrium going on with some of the ice losing water molecules as they gain bits of energy.

The same process actually happens in reverse to form snow flakes. The snow flake is actually formed by this process known as sublimation. Water vapour sublimates to form ice crystals directly without going through a liquid otherwise it would be little balls. It goes straight to form ice crystals and you get the beautiful crystals in snow flakes.

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