residue vs leftover

residue and leftover both are nouns.

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

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

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

Other hyponyms of residue include remnant.

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

residue (adjective)	leftover (adjective)
	not used up