Here's why:

* Substance: A pure substance has a fixed chemical composition and uniform properties throughout. It cannot be separated into simpler components by physical means.

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

Ammonium nitrate (NH₄NO₃) has a specific chemical formula and is composed of only nitrogen, hydrogen, and oxygen atoms. It can't be broken down into simpler components without undergoing a chemical reaction.