Sodium, chloride or table salt is an example of a substance because

Table salt has a uniform and definite composition. It is not a mixture because the sodium and the chlorine are chemically bonded, meaning you cannot take the sodium out of the chlorine or vice versa.

Explanation:

All samples of salt are chemically identical, whereas a mixture can vary. For example, salt water is a mixture because you can have differing amounts of salt or water. The composition is not the same all the way throughout.