caustic vs unpleasant

caustic is a noun but unpleasant is not a noun.

Word	Noun	Adjective	Verb	Adverb
caustic	Yes	Yes	No	No
unpleasant	No	Yes	No	No

caustic (noun)	unpleasant (noun)
the envelope of light rays reflected or refracted by a curved surface or object
any chemical substance that burns or destroys living tissue

caustic (adjective)	unpleasant (adjective)
of a substance, especially a strong acid; capable of destroying or eating away by chemical action	offensive or disagreeable; causing discomfort or unhappiness
harsh or corrosive in tone