If you had used "sodium chloride" as an example, what you say would make sense, though most would regard it as a trivial observation.
However, using "table salt" gives you aa problem as, according to Wikipedia (http://en.wikipedia.org/wiki/Sodium_chloride
),
"Most table salt sold for consumption today is not pure sodium chloride. In 1911, magnesium carbonate was first added to salt to make it flow more freely. In 1924 trace amounts of iodine in form of sodium iodide, potassium iodide or potassium iodate were first added, to reduce the incidence of simple goiter." So not pure and probably varying slightly in composition between different producers.
Is your idea of "substance" more useful that the the chemist's definitions of "element" and "compound".