Edit for follow up question: if a freezer is sealed (or a fridge, or any other container sublimination or even evaporation takes place), how does this gas not build up and cause expanding or even a more violent reaction of said container? Where does it go?
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There’s this super cool thing called sublimation where a solid will turn directly into a gas, skipping the liquid phase altogether. The ice cubes in your freezer slowly sublime away into gas.
Nope. At pretty much any temperature, ice and water will spontaneously turn to gas. When it’s liquid to gas, we call it evaporation. When it’s solid to gas, we call it sublimation.
Being frozen will tend to slow evaporation, but it doesn’t stop it. How fast evaporation occurs will depend on the humidity inside the freezer, if the ice is covered at all, and if it’s where a freezer fan blows air across it.
For OP’s edit/follow-up: sublimation of ice in a freezer is *slow*. The sublimated water will just be water vapour in the air of the freezer. If you manage to get up to 100% humidity in the freezer air and freezer is truly sealed, the sublimation will stop (technically, the rate of ice-to-gas and gas-to-ice will equalize). The amount of water to hit 100% humidity in a really cold environment like a freezer is very small so you’re not talking about massive amounts of water vapour.
In practice, freezers aren’t truly airtight and you open them periodically, which lets out some of the vapour and then sublimation can resume.
If you deal with materials the sublimate fast, like carbon dioxide, they *do* build up and blow up the container. That’s the whole principle behind dry-ice bombs.
Sublimation has nothing to do with it – it has everything to do with the defrost cycle in a freezer.
As you noted, a freezer is essentially an enclosed box. Water vapor in the air freezes out, creating frost along the fridge sides and building up on the ice inside of the ice maker. Each time you open the freezer, water vapor from the outside air gets in, replenishing the water vapor inside of the freezer that had previously turned to frost.
To prevent the buildup of frost, freezers go through a periodic defrost cycle where they briefly allow the freezer to warm up to just above freezing. This melts the frost, which then drains through the freezer’s ventilation system and into an evaporator pan below the fridge. One consequence of this is that each defrost cycle causes a small amount of the ice to melt and then evaporate, shrinking ice cubes over time.
While sublimation is technically a thing, its so slow that you would never notice it on a human timescale and, again, ice builds up in a freezer due to the constant freezing out and reintroduction of water in the air.
The 0°C and 100°C that we all know and love are the melting and boiling points of water **at standard pressure**. This point about pressure is important, and also complicated.
Standard pressure, by the way, means normal room temperature. There’s a specific definition somewhere, but unless you are up a high mountain, where the pressure is much lower, you are probably pretty close to standard pressure right now.
But here’s the thing: the pressure that matters for evaporation is the pressure of *just the water in the air, not all of the air. Air is mostly oxygen, nitrogen, and so on, and just a little bit of water. So the pressure of just the water (called the partial pressure) is much much lower than standard pressure. At that really low pressure, ice at freezer temperatures actually wants to jump straight from solid to gas, skipping liquid water completely. This is called sublimation.
If you’ve got this far, you may be wondering, if this is true, why does water actually boil at 100°C? Didn’t I just make an argument that it should sublimate at much lower temperatures? Well here it gets even more subtle. Only the water/ice right at the surface is experiencing water at that very low partial pressure. Below the surface, the water is surrounded by 100% water or ice at standard pressure, so it really does experience the 100°C point. This is the difference between evaporation and boiling. Evaporation is just at the surface, and occurs even at low temperatures. Boiling is where the water turns to gas all the way through, and only happens at 100°C.
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