chem,係咪所有metal oxide都insoluble

Most of the metal oxides are insoluble in water. Only a few metal oxides reacts with water to form soluble metal hydroxides, and thus the "dissolving" of such metal oxides is due to chemical reactions with water.

The solubility of metal oxides :

(1) All the oxides of Group I metals can react with water to give soluble metal hydroxide solution.
Li2O(s) + H2O(l) → 2LiOH(aq)
Na2O(s) + H2O(l) → 2NaOH(aq)
K2O(s) + H2O(l) → 2KOH(aq)
Rb2O(s) + H2O(l) → 2RbOH(aq)
Cs2O(s) + H2O(l) → 2CsOH(aq)
Therefore, the above metal oxides are said to be "soluble in water" due to formation of solutions of metal hydroxide. In CE level, only Na2O and K2O are important.

(2) For hydroxides of Group II metals, BeO and MgO­ are insoluble in water. All the oxides of other Group II metals react with water to form soluble metal hydroxides, but the solubility of such metal hydroxides is 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)
In CE level, only Ca(OH)2 and Ba(OH)2 are important.

(3) All other metal hydroxides are insoluble in water.

In CE level, You can only remember all K+, NH4+ are soluble in water.
Also, K2O

Group I, II oxides(except BeO )react with water while dissolving
Li2O + H2O →2 LiOH
Na2O + H2O → 2 NaOH
K2O + H2O → 2 KOH
CaO +H2O → Ca(OH)2
MgO + H2O → Mg(OH)2
Some medium reactive metal 's oxide are less soluble
Al2O3 + H2O → Al(OH)3
Some low reactive metal's oxide are insoluble
CuO + H2O →Cu(OH)2

Metal oxides mostly react with water to form metal hydroxide instead of dissolve in it.