They consist of only one type of particles; they can either be elements or compounds.

Elements

It is a substance which contains atoms which are exactly the same. Atoms of one element differ from atoms from another element, it cannot be broken by any chemical change.

Compounds

It is a substance that consists of two or more different elements that are chemically combined together.

Mixtures

It is an impure substance and is a combination of two or more substances still maintaining its properties. They are not chemically combined. They can be separated through physical methods.