For use before a noun, the one-word adjective "leftover" is used, as in "leftover meatloaf" or "leftover tasks." The divided adjective "left over" is not used in this way. It's also important to note that the pronunciation of the two adjectives differ. The one-word adjective has its primary stress on the first syllable, "left." The two-word adjective has its primary stress on the "o" of "over."