* Substance: A substance is a pure form of matter with a definite and constant composition.

* Table salt (NaCl): A compound made of sodium and chlorine.

* Gold (Au): An element, a pure substance that cannot be broken down further.

* Nitrogen (N2): An element, existing as a diatomic molecule.

* Air: Air is a mixture of gases, primarily nitrogen, oxygen, and smaller amounts of other gases. Mixtures do not have a fixed composition, making air not a pure substance.