composure vs placidity

composure and placidity both are nouns.

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

As nouns, placidity is a hyponym of composure; that is, placidity is a word with a more specific, narrower meaning than composure:

Other hyponyms of composure include aplomb, assuredness, cool, poise, sang-froid, quiet, repose, serenity, tranquility, tranquillity.

composure (noun)	placidity (noun)
steadiness of mind under stress	a disposition free from stress or emotion
	a feeling of calmness; a quiet and undisturbed feeling