When iodine is heated, it sublimes. Sublimation is the process in which a solid substance surpasses the liquid phase and turns directly into gas when heated. Solid iodine can still be turned into liquid form by using controlled heat and a precise amount of atmospheric pressure.
The molecular properties of a compound determine its reaction when subjected to heat or cold. Although atoms within an iodine molecule pull closely together by a covalent bond, the molecules have very weak interactions with each other known as van der waals forces. Owing to the fragility of these Intermolecular forces, iodine crystals succumb easily when heated and break into a thick purple colored vapor.