Why are new properties observed during a change of state not signs of a chemical change?

An element or compound's atomic structure must change for there to be a chemical change; this is typically a change in the valence electron distribution (ionic charges and bonding). For instance, when iron and oxygen combine, the electronic structures of the two elements change, forming iron oxide, or rust.

Its chemical structure determines how it behaves as a liquid (or solid), but changing the physical form from solid to liquid is not a chemical change. For example, if we take iron (solid) and heat it until it becomes a liquid, we have not changed the chemical (electronic) structure of the iron.

A change of state only involves the rearrangement of molecules or particles; it does not involve a change in their chemical composition. On the other hand, chemical changes involve the formation of new substances with different chemical properties. Therefore, new properties observed during a change of state are not indicative of a chemical change.