solubility n.

late 14c., "unconstipated;" early 15c., "capable of being dissolved," from Old French soluble "expungable, eradicable" (13c.), from Late Latin solubilis "that may be loosened or dissolved," from stem of Latin solvere "to loosen, dissolve," from PIE *se-lu-, a compound of reflexive pronoun *s(w)e- (for which see idiom) + root *leu- "to loosen, divide, cut apart." The meaning "capable of being solved or explained" is attested from 1705 (Berkeley). Substances are soluble, but not solvable; problems can be either.

word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).

Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]