8th vs eighth

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

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

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

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

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

8th (adjective)	eighth (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