leftover vs left over

leftover is a noun but left over is not a noun.

Word	Noun	Adjective	Verb	Adverb
leftover	Yes	Yes	No	No
left over	No	Yes	No	No

As adjectives, leftover and left over are synonyms defined as:

Other synonyms of leftover include left, odd, remaining, unexpended.

leftover (noun)	left over (noun)
a small part or portion that remains after the main part no longer exists

leftover (adjective)	left over (adjective)
not used up	not used up