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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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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