tranquility vs placidity

tranquility and placidity both are nouns.

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

As nouns, tranquility and placidity are synonyms defined as:

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

tranquility (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