Here's why:

* Substance: A pure form of matter with a definite and constant composition. It cannot be broken down into simpler substances by physical means.

* Mixture: A combination of two or more substances that are not chemically bonded. The components of a mixture can be separated by physical means.

Iron is an element on the periodic table, meaning it's a pure substance that cannot be broken down further. It has a distinct set of properties and is not a combination of other elements.