Sodium metal is a pure substance and so is Sodium chloride, in spite of the fact that Sodium chloride contains two different elements. Explain.

"Pure substance", by definition, means any single type of "material", not necessarily single type of element. Common Salt (Sodium chloride) is a pure substance, as long as it does not have anything else added to it, so is Sodium. A pure substance need not mean that it has to consist of a single element - as long as the entire substance contains only the compound (or element) you are referring to, it can be called a "Pure substance".