Which statement correctly describes whether a compound is a pure substance?   A compound is not a pure substance because each of its molecules is made up of the atoms of two or more different elements. A compound is a pure substance because its molecule cannot be ken down into simpler particles by physical means. A compound is not a pure substance because it is not an element, and only elements are pure substances. A compound is a pure substance because it consists of two different elements, which are pure substances.