sour vs tartness

sour and tartness both are nouns.

Word	Noun	Adjective	Verb	Adverb
sour	Yes	Yes	Yes	No
tartness	Yes	No	No	No

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

Other hyponyms of sour include acerbity, vinegariness, vinegarishness.

As nouns, sour and tartness are synonyms defined as:

sour and tartness: the taste experience when vinegar or lemon juice is taken into the mouth

Other synonyms of sour include sourness.

sour (noun)	tartness (noun)
the property of being acidic	a sharp sour taste
the taste experience when vinegar or lemon juice is taken into the mouth	a rough and bitter manner
a cocktail made of a liquor (especially whiskey or gin) mixed with lemon or lime juice and sugar	the taste experience when vinegar or lemon juice is taken into the mouth

sour (adjective)	tartness (adjective)
having a sharp biting taste
showing a brooding ill humor
inaccurate in pitch
smelling of fermentation or staleness
in an unpalatable state
one of the four basic taste sensations; like the taste of vinegar or lemons

sour (verb)	tartness (verb)
go sour or spoil
make sour or more sour