* Sublimation is the process where a solid changes directly into a gas without going through a liquid phase.

* Physical changes alter the form or appearance of a substance but don't change its chemical composition.

In the case of iodine crystals:

* The iodine molecules themselves remain the same. They are still I₂ molecules.

* The change is simply a shift in state from solid to gas.

Chemical changes involve the formation of new substances with different chemical properties. If iodine crystals were to react with another substance, forming a new compound, that would be a chemical change.