* Element: A pure substance made up of only one type of atom (e.g., gold, oxygen, hydrogen).

* Compound: A substance formed when two or more different elements chemically combine in a fixed ratio (e.g., water, salt, carbon dioxide).

* Mixture: A combination of two or more substances where each substance retains its own properties (e.g., salt and pepper, air).

Water is made up of two elements: hydrogen and oxygen, chemically bonded together in a fixed ratio of 2:1 (H₂O). This fixed ratio and the chemical bond make water a compound, not a mixture.