Why can $O^{2-}$ not exist in water?
a) It is a very strong base which reacts rapidly to form $OH^-$ b) It reacts rapidly with water to form $H_3O^+$ c) It realises an equilibrium with $OH^-$ d) It reacts with $H_2 (g)$ producing $H_2O$ and is therefore destroyed
The right answer is: a) It is a very strong base which reacts rapidly to form $OH^-$