acid vs acidulous

acid is a noun but acidulous is not a noun.

Word	Noun	Adjective	Verb	Adverb
acid	Yes	Yes	No	No
acidulous	No	Yes	No	No

As adjectives, acid and acidulous are synonyms defined as:

Other synonyms of acid include acidic, acidulent.

acid (noun)	acidulous (noun)
street name for lysergic acid diethylamide
any of various water-soluble compounds having a sour taste and capable of turning litmus red and reacting with a base to form a salt

acid (adjective)	acidulous (adjective)
having the characteristics of an acid	being sour to the taste
being sour to the taste
harsh or corrosive in tone