It is my understanding that when ice cubes “shrink” in the freezer over time, it is due to sublimation. How does sublimation work exactly? What causes the change in state?
In: Chemistry
Sublimation is going from solid to gas without becoming a liquid. One at a time, or so, water molecules get enough energy to pop loose from the outer edge of the cube, and the fans and self-defrosting mechanism in the freezer drags them away before they can re-attach to the surface.
There is no sublimation. Sublimation is changing from solid to gas without going through a liquid phase. To my knowledge carbon dioxide (CO2) when frozen (dry ice) will sublimate when changing from solid to gas. Water will actually swell or increase in volume when freezing.
Any shrinkage is like the water content of whatever is freezing escaping and manifesting as ice crystals on the surface (freezer burn) resulting in shrinkage of the main item. At least that what I think happens.
It’s basically the same as the process by which a piece of bread goes stale except with a block of ice it happens much slower – but also much faster than it happens with a rock. Sublimation is just evaporation at a lower temperature.
The same way it evaporates from a room temperature cup. Ice has a non-zero vapor pressure, so there will always be some small amount evaporating off.
It works the same as a glass of water evaporating even though it is below its boiling point. There is always a vapor pressure which is kind of a measure of how badly a liquid or solid wants to turn into a gas.
When the vapor pressure is equal to the atmospheric pressure, you get boiling. But ultimately, unless you have a perfectly sealed vessel, the substance will continue to have molecules turn gaseous and fly away.
Even metals and what not will have vapor pressures, but they would be so stupidly low that I can’t adequately fathom it. Like 1 atom out of trillions upon trillions per year or something silly.
Temperature is an average. Some molecules of ice are higher than average, boiling. And those boil away.
The other thing to add besides sublimation is that fridge/freezer combos basically always have a defrost cycle which activates a heating element behind the freezer to melt any ice that otherwise would build up in the freezer and need to be chipped or melted away. While ice in a cube tray will melt into the tray and refreeze, this increase in temperature and phase change increases the rate of evaporation.
If you have a freezer/fridge combo and a chest freezer do an experiment with ice cube trays. Fill them both up, put one in each freezer and leave it alone for a couple weeks and compare the two.
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