Must a new substances be formed when you observe a chemical property? explain.

Yes! a new substance is formed when you observe a chemical property. 

A chemical property changes the identity of the original item. Such as rust forming on an iron nail from oxygen reaction.

Chemical properties are those that have the faculty to change the nature of the substance, i.e. to transform the substance (element or compound) into a different substance.
For example, Metals have the property to combine with oxygen to produce the metallic oxide.
This property drives to the change of the metal into a new product (with different composition, different bonds, different molar mass, etc).