tranquillity vs placidity

tranquillity and placidity both are nouns.

Word	Noun	Adjective	Verb	Adverb
tranquillity	Yes	No	No	No
placidity	Yes	No	No	No

As nouns, tranquillity and placidity are synonyms defined as:

Other synonyms of tranquillity include quiet, repose, serenity, tranquility.

tranquillity (noun)	placidity (noun)
a disposition free from stress or emotion	a disposition free from stress or emotion
a state of peace and quiet	a feeling of calmness; a quiet and undisturbed feeling
an untroubled state; free from disturbances