h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Any physical chemists out there?

If I leave a container of stew in my freezer for a long time - a year or more - water from the stew evaporates from the ice, and then condenses on the lid forming ice crystals.

How does this happen? The freezer is at -18°C so there should be no evaporation. At least that's what I was taught. Does evaporation (or possibly sublimation) actually happen at all temperatures?

I've noticed Ice formation inside containers, but I've not seen any corresponding reduction of volume in the intended contents.

I've no explanation for it either...

All solids have an equilibrium with a vapour form at their surface. The extent of this is called the 'vapour pressure'. That is - molecules will evaporate and then condense back onto the solid again continuously. Liquids also have this to a much greater extent.

Some - say granite or some other rock - have this so negligibly that you will never see it 'evaporate' in billions of years.
But some - like ice - have a relatively high vapour pressure.

This is how freeze drying works - you put the water there into such a cold temperature (usually liquid nitrogen temp in our labs) - that the vapour pressure is very high (you can see it 'smoking') - then put it under vacuum so that the vapour is sucked away to a condenser elsewhere. The latent heat of evaporation at the surface (i.e the energy required for solid molecules to debind from the crystal lattice surface) is lost from the solid and so this keeps it cold and frozen as this process continues until there is no solid water left and all you are left with is whatever was dissolved or suspended in it.

Thankyou Orcus

You're most welcome

Easy one for me for once lol

That makes it very clear, thanks. I suspected it was something like that but never saw it explained anywhere.

You've got one of those new-fangled frost-free freezers. The nano-robots that do the defrosting can't tell the difference between that irritating, drawer-jamming hoar frost and your ice-cubes (stew, whatever).

Ice, in principle, can evaporate - this phenomenon of direct transition from the solid phase g is called sublimation (sublimation). It is due to sublimation the presence of the smell of solids - naphthalene, iodine and others. So the very fact of evaporation is not a question. Question - in contrast to the design of refrigerators.
Modern refrigerators are almost everything - with no frost technology. that is, in the freezer the ice does not freeze onto the walls. This is done by a special mode of operation of the freezer - from time to time the back wall is heated there and air is pumped out. However, the steam that is there. In the old refrigerators there was no such forced ventilation of the freezer - on the contrary, due to the fact that there was an air exchange, even if it was inferior, with the external environment, moisture from the surrounding air cracked there.
Read more here.Thanks.I hope I gave you the answer to the question

The defrosting cycle in frost-free freezers(with fans etc) reduces the actual humidity of the space or as eloquently put earlier, the ice will sublimate in an attempt to equalize the difference between the solid surface and the lower free space. In closed containers that free-roaming moisture is drawn to the coldest surface hence the frost on the lid.In fan forced or natural convection plate freezers that free-roaming water molecules land on the evaporator coil which is substantially colder due to the refrigeration process. So, in short, if you want to keep the moisture content stable in anything stored in a freezer,seal it tight in an airtight container with minimal free air space (but allow for expansion of ice as the moisture freezes).

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How I love this kind of threads! Thank you!