Final answer:
MgO (s) and H₂O (l) are compounds, while O₂ (g) is an element. Compounds consist of two or more different types of atoms chemically bonded, while elements consist of only one type of atom.
Step-by-step explanation:
Classification of Substances
The substances MgO (s), H₂O (l), and O₂ (g) can be classified based on their chemical composition. MgO, which stands for magnesium oxide, is a compound because it consists of magnesium and oxygen atoms bonded together in a fixed ratio. H₂O, better known as water, is also a compound made up of two hydrogen atoms and one oxygen atom in a fixed ratio. O₂ represents molecular oxygen, which is an element because it is composed of only one type of atom, oxygen, in this case, making it a pure substance.