Pure substances are defined as a particle that is made up only one type of particle. Examples of pure substances are atoms, elements, compounds, and molecules. Atoms are the smallest unit of all matter that are composed of three subatomic particles- positive protons, neutral neutrons, and negative electrons. Elements are atoms with a specific number of protons. A compound is a substance that is made of two or more different elements chemically combined, whereas a molecule is a substance that is made of two or more atoms of the same element.
The chemical formula for water is H2O. Thus, water is made of two hydrogen atoms and one oxygen atom. Since water contains two different elements, it would be considered a compound.