Why iodine can skip the puddle stage
Warm a solid sample of iodine, and its dark crystals can give way to a vivid purple gas without first forming a visible liquid. The change is called sublimation: a solid turns directly into gas. When the vapor cools, the reverse process—deposition—can build solid crystals again without a liquid phase in between.
Molecules escape from the crystal
In solid iodine, pairs of iodine atoms, called molecules, are packed into an orderly crystal. Heating gives those molecules more energy. Some at the surface gain enough to break away from the attractions holding the crystal together and move freely through the air as gas.
A substance does not need to melt before it can evaporate. Melting happens when the solid’s structure gives way to a liquid; sublimation happens when molecules leave the solid directly. Which change is easiest depends on temperature and pressure, as well as the strength of the attractions between molecules. Under ordinary conditions, iodine can produce a noticeable vapor before a puddle forms.
The color travels with the vapor
Iodine vapor looks purple because gaseous iodine molecules absorb some colors of visible light and let others pass through to the eye. That color is evidence of iodine molecules in the gas, not smoke from burning: heating alone can produce the vapor.
As the gas reaches a cooler surface, its molecules lose energy and gather into a solid crystal. The same sample can therefore make a striking round trip—from crystal to purple gas and back—without passing through a liquid stage.
