Eli5: why do Ice cubes get smaller when left in the freezer for an extended period

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Eli5: why do Ice cubes get smaller when left in the freezer for an extended period

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Water can sublimate and go directly from a solid to a gas. Many freezers are frost free so the low humidity removes ice.

They evaporate. Yep, that’s right, they evaporate. Water can evaporate when frozen. As all it takes for evaporation to happen is dry air. And the freezer is full of dry air. So as the dry air passes over the water, even in a frozen state, it picks up some of it and carries it off.

Obviously it doesn’t happen as fast as like water on a hot day spilled on the sidewalk, but it still happens.

So over a very long time, frozen water shrinks.

When a solid goes directly to a gas without being a liquid it is said to *sublime*. Or sublimate is cool now, too, I think. It depends upon air pressure too and on Mars, which is nearly a vacuum, some of the first ice cubes we found [disappeared on camera](https://www.youtube.com/watch?v=VwdX8UELj6c).

https://en.wikipedia.org/wiki/Phoenix_(spacecraft)#Presence_of_shallow_subsurface_water_ice

Most freezers are “self defrosting” or “frost free”. These cycle through short periods of warming to melt ice build up. Without this, freezers fill with ice that is moisture pulled from the air.

The answers attributing this to sublimation are incorrect. You can see from the [phase diagram of water](https://en.m.wikipedia.org/wiki/Phase_diagram) that at freezer conditions, the ice will melt not sublimate.

What is likely happening is that the ice is melting and reforming somewhere else (like the bottom of the ice tray). This is caused for either a defrosting cycle or the ice just changing shape.

Note: ice in steady state will leave the same amount of water frozen but not necessarily the exact same molecules. e. g. Ice can gain a frozen molecule on the bottom without removing heat from the system and long as it loses one on the top. This is why ice cubes fuse together in your glass of water even though the overall temperature of your drink is “too warm” to form ice. Over time this process can change the shape of the ice without actually losing any ice.