Why are new properties observed during a change of state not signs of a chemical change?
A Change of State is not chemical change, but a physical one.
To be a chemical change, the atomic structure of an element or compound must change. This is usually a change in the valence electron distribution (ionic charges and bonding). For example, combining iron with oxygen results in a change in the electronic structure of both iron and oxygen, resulting in a different compound – iron oxide, or rust.
However, if we simply take iron (solid) and heat it until it becomes a liquid we have not changed the chemical (electronic) structure of the iron. HOW it behaves as a liquid (or solid) is determined by its chemical structure. But changing the physical form from solid to liquid is not a chemical change.