leftover vs residue

leftover and residue both are nouns.

Word	Noun	Adjective	Verb	Adverb
leftover	Yes	Yes	No	No
residue	Yes	No	No	No

As nouns, residue is a hypernym of leftover; that is, residue is a word with a broader meaning than leftover:

leftover: a small part or portion that remains after the main part no longer exists
residue: something left after other parts have been taken away

Other hypernyms of leftover include balance, remainder, residual, residuum, rest.

leftover (noun)	residue (noun)
a small part or portion that remains after the main part no longer exists	something left after other parts have been taken away
	matter that remains after something has been removed

leftover (adjective)	residue (adjective)
not used up