A metal surface becomes dull on exposure to air is a chemical change. A physical change is ice melting or breaking. It is still the same chemical (H20). Also if a metal surface got scratched it is a physical change. Still the same chemical, just scratched up. But the metal surface reacting to to the air is a chemical change. For example copper, when exposed to air turns green. It is not just pure copper any more. It now has a different chemical structure. The green is a copper oxide.