What two kinds of matter are pure substances?
A pure substance is one in which the composition is definitive and constant throughout, and that has distinct chemical properties. These substances are comprised of only one entity, not a mixture of two or more.
Elements are pure substances, assuming they are free of imperfections, because they are made of only one type of atom.
Compounds are also pure substances, like water, table salt, table sugar, etc. because they are comprised of one type of compound (