When solid Iodine is heated,it changes to purple vapour which solidifies on the cooler parts of the boiling tube.

The process of change of state from solid to gas and from gas to solid without passing through the liquid state is called Sublimation.

Other examples of solids which sublime on heating include  Ammonium Chloride , Solid Carbon (IV) Oxide, Iron (III) Chloride and Aluminium Chloride