Here's why:

* Pure substances have a fixed chemical composition and uniform properties throughout. They cannot be separated into simpler substances by physical means.

* Elements are the simplest form of matter and cannot be broken down into simpler substances. They are made up of only one type of atom (e.g., gold, oxygen, carbon).

* Compounds are formed when two or more elements chemically combine in a fixed ratio (e.g., water (H₂O), table salt (NaCl)).

In contrast, mixtures are combinations of two or more substances that are not chemically bonded and can be separated by physical means.