tap water vs H2O

tap water and H2O both are nouns.

Word	Noun	Adjective	Verb	Adverb
tap water	Yes	No	No	No
H2O	Yes	No	No	No

As nouns, H2O is a hypernym of tap water; that is, H2O is a word with a broader meaning than tap water:

tap water: water directly from the spigot
H2O: binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade; widely used as a solvent

Other hypernyms of tap water include water.

tap water (noun)	H2O (noun)
water directly from the spigot	binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade; widely used as a solvent