Sodium chloride, better known as table salt, is considered a pure substance because it is a homogeneous compound. A pure substance can be either an element or a compound, where the individual chemical elements are always in a fixed proportion to each other.
In a pure substance, the chemical and physical properties are always the same in any given sample. For instance, table salt is always made of one molecule of sodium bonded to one molecule of chlorine. Other examples of pure substances include water, sodium bicarbonate and sucrose, also known as pure table sugar. Water is a pure substance because the chemical composition always consists of two hydrogen atoms bonded to one oxygen atom.