We use the same prefix “im-” for both impossible and impossibility, but why do we use “un-” for unstable but “in-” for instability?
I did a bit of internetting to see if I could find a reasonably believable answer. Turns out the word “unstable” dates back to the 13th century and borrows from Old English and Old French, both Germanic languages. Germanic languages use the “un-” prefix.
“Instability,” on the other hand, is slightly newer and dates back to the 15th century. It borrows from Latin; Latin uses the “in-” (or “im-”) prefix.
Both “impossible” and “impossibility” borrow from Latin, hence the same “im-” prefix.
The more you know!