@justforthelulz
It's been a while since I've had a chemistry class, but I think that rather than a level of acidity, it depends on the type acid. It needs to be some acid that reacts with silver to form a compound that's liquid at the soup's temperature or something that could be soluble in liquids. A quick search brings up nitric acid, which reacts together to form silver nitrate, a salt that dissolves in water.
On the other hand, the melting point of a silver spoon is 961.8 degrees Celcius, which is well in the range of the temperature of magma / lava.