* Iron is a pure element, meaning it has a uniform composition throughout.

* Rust (iron oxide) is a compound formed when iron reacts with oxygen and water. The rust is a distinct layer on the surface of the iron, creating two separate phases.

Heterogeneous mixtures have components that are not evenly distributed and can be visually distinguished. In the case of iron and rust, you can clearly see the difference between the shiny iron and the reddish-brown rust layer.