pure substance: is a phase of uniform composition and unchanging can not be decomposed into other substances of different classes by physical methods.
elements: they are pure substances that can not be decomposed into simpler ones through normal chemical processes.
compounds: they are pure substances made up of two or more elements, which can be decomposed by chemical methods on the elements of which are constituted. A compound always has the same elements and the same proportions regardless of the process followed.
Blended is an aggregation of different substances without causing any chemical reaction between them and therefore may be separated by physical methods.