afire vs on fire

afire and on fire both are adjectives.

Word	Noun	Adjective	Verb	Adverb
afire	No	Yes	No	No
on fire	No	Yes	No	No

As adjectives, afire and on fire are synonyms defined as:

Other synonyms of afire include ablaze, aflame, aflare, alight.

afire (adjective)	on fire (adjective)
lighted up by or as by fire or flame	lighted up by or as by fire or flame