fixity vs unchangeableness

fixity and unchangeableness both are nouns.

Word	Noun	Adjective	Verb	Adverb
fixity	Yes	No	No	No
unchangeableness	Yes	No	No	No

As nouns, unchangeableness is a hypernym of fixity; that is, unchangeableness is a word with a broader meaning than fixity:

fixity: the quality of being incapable of mutation
unchangeableness: the quality of being unchangeable; having a marked tendency to remain unchanged

Other hypernyms of fixity include changelessness, unchangeability, unchangingness.

fixity (noun)	unchangeableness (noun)
the quality of being fixed in place as by some firm attachment	the quality of being unchangeable; having a marked tendency to remain unchanged
the quality of being incapable of mutation