left over vs leftover

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

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

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

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

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

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