like not melting to water but an ice cube left out side will slowly get smaller even when its super cold out.
In: 5
You’re probably witnessing *sublimation*. That is the state change where solids skip melting and turn directly into a gas. This happens for the same reason that puddles will evaporate without boiling.
Temperature is a measure of the *average* energy of the molecules in the sample. Some will be above that average and some will be below it. As the molecules jiggle around, they trade some of their energy back and forth, knocking into each other like billiard balls. Sometimes, randomly, some of them will knock into each other in such a way that one molecule ends up with a lot more energy.
If that molecule is inside the sample, nothing much happens. It will bang into another molecule or molecules and lose that energy and that’s it. If, however, that molecule is on the surface it might be given enough energy to be knocked away. If the energy is enough to loosen the molecule and it “melts” out of the solid structure, but doesn’t fly off as a gas, then it won’t get very far before it bumps into one of the frozen molecules and trade that energy back in, and get trapped again. If it has enough to fly off as a gas, it will take that energy with it and just like that your ice cube is one molecule smaller.
Gaseous molecules can also bang into the solid, lose their energy, and become trapped. If there is a lot of moisture in the air, some of those water molecules will run into the ice and become trapped, and the ice cube is one molecule *bigger*. In static conditions, this will lead to an equilibrium where there are just as many molecules getting trapped as there are molecules becoming freed. If the air is very dry, then it will take a long time to reach that equilibrium and the ice will shrink a lot. If the air has a high relative humidity, the ice won’t shrink, or it may even grow a bit.
The equilibrium also depends on the overall temperature of the sample and the air. If everything is very cold, very few molecules will end up with enough energy to leave, and it will be more likely for gaseous molecules to get stuck. If it’s not *super* cold, the equilibrium will shift more towards the ice sublimating rather than the gas condensing.
Freezers tend to be cold, but also *very* dry. Moisture in the air tends to condense onto the coils that cool the air in the freezer, and the system is designed to remove that moisture 1) because it preserves your food better, since bacteria like moisture, and 2) condensation is bad for the coils. Since your refrigerator stays closed most of the time, no new moisture can get in so the freezer stays very dry, shifting the equilibrium so that ice tends to sublimate and shrink. This also causes “freezer burn” where foods tend to become dry and tough, as the water sublimates away.
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