Here's why:

* Pure Substances: Have a fixed and definite composition. They are made up of only one type of molecule or atom.

* Mixtures: Contain two or more different substances that are not chemically bonded.

Sodium phosphate (Na₃PO₄) has a specific chemical formula. It always contains the same ratio of sodium (Na), phosphorus (P), and oxygen (O) atoms. This consistent composition makes it a pure substance.