serenity vs placidity

serenity and placidity both are nouns.

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

As nouns, serenity and placidity are synonyms defined as:

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

serenity (noun)	placidity (noun)
a disposition free from stress or emotion	a disposition free from stress or emotion
the absence of mental stress or anxiety	a feeling of calmness; a quiet and undisturbed feeling