A pure substance is an element or compound. If it's an element, then it's made of only one kind of atom. If it's a compound, then it's made of only one kind of formula unit. For example, hydrogen (H2) and oxygen (O2) are elements. In pure samples of H2 and O2, you find only one kind of element. H2O is a compound, water. In pure samples of H2O, you find only formula units of H2O (called molecules because H2O is covalent). Neither the components that make up H and O atoms, nor the atoms that make up H2O can be separated by physical means. H2, O2, and H2O are all considered substances.