sad vs sadness

sad is not a noun while sadness is a noun.

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

sad (noun)	sadness (noun)
	the quality of excessive mournfulness and uncheerfulness
	emotions experienced when not in a state of well-being
	the state of being sad

sad (adjective)	sadness (adjective)
experiencing or showing sorrow or unhappiness
bad; unfortunate
of things that make you feel sad