Here's why:

* Pure Substance: A pure substance is made up of only one type of molecule.

* Water Molecule: Liquid water is composed of H₂O molecules, meaning each molecule contains two hydrogen atoms and one oxygen atom.

While water can contain impurities like dissolved minerals or gases, the water itself is still considered a pure substance because it's made up of only water molecules.

Let me know if you have any other questions! 😊