eighth vs 8th

eighth is a noun but 8th is not a noun.

Word	Noun	Adjective	Verb	Adverb
eighth	Yes	Yes	No	No
8th	No	Yes	No	No

As adjectives, eighth and 8th are synonyms defined as:

eighth and 8th: coming next after the seventh and just before the ninth in position

eighth (noun)	8th (noun)
one part in eight equal parts
position eight in a countable series of things

eighth (adjective)	8th (adjective)
coming next after the seventh and just before the ninth in position	coming next after the seventh and just before the ninth in position