* Density is mass per unit volume.

* Oxygen (O2) is a diatomic molecule. This means each oxygen molecule has two oxygen atoms, making it heavier than a hydrogen molecule.

* Hydrogen water (H2O) is water. Water is made up of two hydrogen atoms and one oxygen atom.

* Even though water has an oxygen atom, it's still less dense than pure oxygen. This is because the volume of a water molecule is larger than the volume of an oxygen molecule.

In summary: Oxygen is denser than hydrogen water due to the heavier mass of oxygen molecules compared to water molecules.