Other forms: acidities
Acidity is sourness, so you might turn down a bite of your friend's kumquat because its acidity is just too much for you.
This noun is good for literal acidity, like a lemon's sour flavor or, in scientific terms, the chemical quality of a substance that reacts with a base to make salt. You can also use it in a figurative way when someone's tone is sharp and biting: "The acidity in my dad's voice told me we were really in trouble for eating all the tomatoes in his garden."