sadness vs mourning

sadness and mourning both are nouns.

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

As nouns, mourning is a hyponym of sadness; that is, mourning is a word with a more specific, narrower meaning than sadness:

Other hyponyms of sadness include bereavement, poignance, poignancy.

sadness (noun)	mourning (noun)
the quality of excessive mournfulness and uncheerfulness	the passionate and demonstrative activity of expressing grief
emotions experienced when not in a state of well-being	state of sorrow over the death or departure of a loved one
the state of being sad

sadness (adjective)	mourning (adjective)
	sorrowful through loss or deprivation