sing vs choir

sing is not a noun while choir is a noun.

Word	Noun	Adjective	Verb	Adverb
sing	No	No	Yes	No
choir	Yes	No	Yes	No

As verbs, choir is a hyponym of sing; that is, choir is a word with a more specific, narrower meaning than sing:

sing (noun)	choir (noun)
	the area occupied by singers; the part of the chancel between sanctuary and nave
	a chorus that sings as part of a religious ceremony
	a family of similar musical instrument playing together

sing (verb)	choir (verb)
make a whining, ringing, or whistling sound	sing in a choir
divulge confidential information or secrets
to make melodious sounds
produce tones with the voice
deliver by singing