有咩oxide soluble in water 有咩oxide insoluble

A metal oxide is soluble in water only if it reacts with water to give a soluble hydroxide. Therefore, there is no oxide ion in the solution when a metal oxide is dissolved in water.

All Group I oxides react with water to give soluble hydroxides.
Li2O(s) + H­2O(l) → 2LiOH(aq)
Na2O(s) + H­2O(l) → 2NaOH(aq)
K2O(s) + H­2O(l) → 2KOH(aq)
Rb2O(s) + H­2O(l) → 2CsOH(aq)
Cs2O(s) + H­2O(l) → 2CsOH(aq)

BeO and MgO are insoluble in water. All other Group II oxides react with water to give soluble hydroxides. However, the solubilities of these soluble Group II hydroxides are low.
CaO(s) + H2O(l) → Ca(OH)2(aq)
SrO(s) + H2O(l) → Sr(OH)2(aq)
BaO(s) + H2O(l) → Ba(OH)2(aq)

All oxides of elements in groups other than Group I and Group II are insoluble.