acerb vs caustic

acerb is not a noun while caustic is a noun.

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

As adjectives, acerb and caustic are synonyms defined as:

Other synonyms of acerb include acerbic, acid, acrid, bitter, blistering, sulfurous, sulphurous, virulent, vitriolic.

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

acerb (adjective)	caustic (adjective)
sour or bitter in taste	of a substance, especially a strong acid; capable of destroying or eating away by chemical action
harsh or corrosive in tone	harsh or corrosive in tone