9 vs ix

9 is a noun but ix is not a noun.

Word	Noun	Adjective	Verb	Adverb
9	Yes	Yes	No	No
ix	No	Yes	No	No

As adjectives, 9 and ix are synonyms defined as:

9 and ix: denoting a quantity consisting of one more than eight and one less than ten

Other synonyms of 9 include nine.

9 (noun)	ix (noun)
the cardinal number that is the sum of eight and one

9 (adjective)	ix (adjective)
denoting a quantity consisting of one more than eight and one less than ten	denoting a quantity consisting of one more than eight and one less than ten