sad vs bittersweet

sad is not a noun while bittersweet is a noun.

Word	Noun	Adjective	Verb	Adverb
sad	No	Yes	No	No
bittersweet	Yes	Yes	No	No

sad (noun)	bittersweet (noun)
	twining shrub of North America having yellow capsules enclosing scarlet seeds
	poisonous perennial Old World vine having violet flowers and oval coral-red berries; widespread weed in North America

sad (adjective)	bittersweet (adjective)
experiencing or showing sorrow or unhappiness	tinged with sadness
bad; unfortunate	having a taste that is a mixture of bitterness and sweetness
of things that make you feel sad