Here's why:

* Fixed Composition: It always has a fixed ratio of sodium and chlorine atoms (1:1).

* Uniform Properties: All samples of pure sodium chloride have the same chemical and physical properties.

* Not Separable by Physical Means: You cannot separate sodium and chlorine from each other using physical methods like filtration or evaporation.

While you might find impurities in table salt, like iodine or anti-caking agents, pure sodium chloride itself is a compound, which is a type of pure substance.