sing vs descant

sing is not a noun while descant is a noun.

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

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

sing (noun)	descant (noun)
	a decorative musical accompaniment (often improvised) added above a basic melody

sing (verb)	descant (verb)
make a whining, ringing, or whistling sound	sing by changing register; sing by yodeling
divulge confidential information or secrets	talk at great length about something of one's interest
to make melodious sounds	sing in descant
produce tones with the voice
deliver by singing