Final answer:
The smallest subdivision of pure H2O that maintains its properties is the molecule. Breaking down water to individual hydrogen or oxygen atoms results in substances with different properties. Further subdivision into protons eliminates the substance's properties entirely.
Step-by-step explanation:
The limit of physical subdivision of pure H2O is the molecule. This is because a molecule of water comprises two hydrogen atoms bonded to one oxygen atom (H2O). An H2O molecule retains the properties of water, such as being liquid at room temperature and being a universal solvent. If you break down water beyond the molecular level, such as to a hydrogen atom or an oxygen atom, you would no longer have water, but rather individual elements with different properties.
Moreover, if you were to break down a hydrogen atom further into subatomic particles like protons, the atom would lose the unique properties of hydrogen. As a physical property, states of matter differ on an atomic level where water exists in three forms: as a solid (ice), a liquid (water), and a gas (water vapor).