* Element: An element is a substance made up of only one type of atom (like oxygen or hydrogen).

* Compound: A compound is formed when two or more different elements chemically combine in a fixed ratio. Water (H₂O) is made of two hydrogen atoms and one oxygen atom bonded together.

* Solution: A solution is a homogeneous mixture of two or more substances. While water can be used to dissolve other substances, pure water itself is not a mixture.

* Heterogeneous mixture: A heterogeneous mixture has components that are not uniformly distributed. Pure water has a consistent composition throughout, making it homogeneous.

Therefore, pure water is a compound because it is formed by the chemical combination of hydrogen and oxygen.