The snow in winter isn’t melting if it’s below freezing; it is actually going straight off into the air as water vapor, without having to melt into liquid water first.
We might be tempted to say that the snow is evaporating, but scientists prefer to reserve the word “evaporation” for liquids only. So when a solid “evaporates” they call the process sublimation. In our everyday experience, we rarely notice solids subliming because sublimation is generally a much slower process than the evaporation of liquids.
Here’s how sublimation takes place. The molecules at the surface of a chunk of solid are not attached as firmly as are the molecules within the bulk of the piece. While the molecules in the bulk are bonded to their brethren in all directions, top, bottom and all around, the surface molecules are bonded in every direction but their “tops,” which are exposed to the great outdoors. They are missing a bit of adhesion to the rest of the solid.
If you consider that molecules are always jiggling around to some extent, it is not too hard to imagine that an occasional surface molecule might break loose and fly off into the air. That molecule has sublimed. The molecules of liquids are more loosely tied together than are the molecules of solids, so the probability that a liquid molecule will break away is much greater. That’s why liquids generally evaporate much faster than solids sublime.
Snow is a great candidate for sublimation because it is made of intricate, lacy crystals with large surface areas; and the more surface molecules there are, the more molecules can sublime. But you can even see solid chunks of ice sublime. Ever notice how old ice cubes shrink in the freezer?
Different solids have different tendencies to sublime because they are made of different atoms or molecules that are tied together with different strengths. Fortunately, the atoms of metals are tied together very tightly, so gold and silver do not evaporate at all.
On the other hand, the molecules of some organic solids are tied together rather loosely, so they have a substantial tendency to fly off as vapor. Moth crystals and deodorizing cakes are usually made of paradichlorobenzene, an organic solid that is a sublime sub-limator, so to speak. Its strong-smelling vapor quickly fills the air and kills both moths and our ability to smell foul odors.
Measure the length of a convenient icicle during a cold spell. Then come back in a day or two and measure it again. Make certain that the temperature hasn’t gotten above freezing in the meantime, so that there hasn’t been any melting. You will see that the icicle has become smaller by sublimation.
