The formation of rust represents a chemical change. When a chemical change occurs, the substance or substances present at the beginning are no longer present at the end of the change. Once a chemical change occurs, it theoretically cannot be undone.
Conversely, when a physical change occurs, the material that is the subject of the physical alteration is unchanged. A physical change theoretically is capable of being undone. One example of this is melting an ice cube; once the ice cube melts, the material (water) remains the same and can be refrozen back into a solid. The rust that forms as the result of a chemical change, on the other hand, is a different type of material than the original object that underwent the transformation. A change has occurred at the atomic level that cannot be undone.