caustic vs acerb

caustic is a noun but acerb is not a noun.

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

As adjectives, caustic and acerb are synonyms defined as:

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

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