* Physical Change: A physical change alters the form or appearance of a substance but doesn't change its chemical composition.

* Sublimation: Sublimation is the process where a solid directly transforms into a gas without passing through the liquid phase.

In the case of iodine, it changes from a solid (purple crystals) to a gas (purple vapor). The chemical composition of the iodine (I₂ molecules) remains the same; it's just changing its physical state.

Here's why it's not a chemical change:

* No new substances are formed: Iodine gas is still iodine. It hasn't reacted with anything to create a different compound.

* The process is reversible: If you cool the iodine vapor, it will condense back into solid crystals. This reversibility is a hallmark of physical changes.

In summary, iodine subliming is a physical change because it involves a change in state without altering the chemical composition of the substance.