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 in the crystal. 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, other water molecules will rejoin onto the ice, and this is what's called a dynamic equilibrium.

So, 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, since we've got a lot of something (water in the form of ice) it will slowly diminish and shrink. It will slowly disappear.

Some water goes in from time to time, for instance if you open the door and you've got a stuffy room. But, basically, it's because there's a dynamic equilibrium going on with some of the ice losing water molecules as they gain energy.

The same process actually happens in reverse to form snow flakes. The snow flake is actually formed by this process known as deposition. Water vapour deposition forms 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.

Comments

Following your explanation of why icecubes sublimate in a Freezer, why does the frost that forms in the freezer not sublimate as well ? It would be great if it did as it would stop me having to de-frost my freezer.

The ice forming all over the freezer does sublime, it's just that the rate of loss is out-paced by orders of magnitude by rates of accumulation, so the net effect is a build-up of ice.

Thanks Chris,
If the rate of deposition is greater than the rate of sublimation this should mean my ice cubes grow not disappear.

In my freezer the ice cubes disappear and the trays are left empty - no frost or anything replaces them unlike the rest of the Freezer. Most but not all the frost appears on the metal surfaces which contain the cooling coils. Does the frost preferentially deposit on the coldest surface ? Is this the reason the plastic ice cube trays remain frost free as they are marginally warmer ?

I'll have to think about that!

I think it is time for an experiment - I will half fill my ice cube tray with water and the other half with frost carefully scraped of the inside of my freezer - hope there is enough I have recently de-frosted it. I will then leave it for a few weeks and record at suitable intervals what is happening to the icecubes and the frost.

I also noticed yesterday that there is frost / ice inside some of the draws that are also plastic so it may not appear to be the plastic that causes nore sublimation than deposition.

I filled 8 of the 20 compartments in my ice tray with water and filled 1 full of ice scrapings from the shelves and draws. A second compartment was filled about 3/4 full as I ran out of ice. The ice in these two compartments had lots of air spaces due to the shape of the scrapings. I filled this on the 11th April and by the 31st most of the ice scrapings had sublimed leaving only a small trace in the bottom of the tray together with some 'dirt' from the scraping.
Some of the rest of pure ice cubes appear to have shrunk slightly and others look almost unchanged.
My conclusion is that Ice that was previously deposited on the shelves and draws seems to sublime at a similar rate to that of solid ice.
That still leaves the mystery of why the ice sublimes from the ice cube tray but not from the inside of the freezer.
A better experiment would have more ice scrapings, possibly mechanically compacted. Also the individual compartments should be weighed during the experiment to allow the rate of sublimation to be measured. Anyone willing to cut up their ice tray in the interests of science?
I have some photographs to support this but cannot post these here.

I thought Sublimation only referred to movement from Solid to Gas State without passing through liquid. I.e. raised to a higher status. Snowflakes are formed by ?Deposition rather than Sublimation?

Ice crystals forming is deposition. The first answer poster has the terms wrong.

Thanks for spotting that; I have fixed it!

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