bitterness vs rancour

bitterness and rancour both are nouns.

Word	Noun	Adjective	Verb	Adverb
bitterness	Yes	No	No	No
rancour	Yes	No	No	No

As nouns, bitterness and rancour are synonyms defined as:

Other synonyms of bitterness include gall, rancor, resentment.

bitterness (noun)	rancour (noun)
the property of having a harsh unpleasant taste	a feeling of deep and bitter anger and ill-will
a rough and bitter manner
the taste experience when quinine or coffee is taken into the mouth
a feeling of deep and bitter anger and ill-will