有咩 Metal oxide 加水會變 Hydroxide?

All oxides of Group I metals (alkali metals) can react with water to give metal hydroxides.

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)

Athough Fr2O is expected to react with water to give FrOH, no such experiment has been done due to the high radioactivity of Fr.


The oxides of some (Ca, Sr, Ba) Group II metals (alkaline earth metals) can react with water to give metal hydroxides.

CaO(s) + H2O(l) → Ca(OH)2

SrO(s) + H2O(l) → Sr(OH)2

BaO(s) + H2O(l) → Ba(OH)2

MgO usually does not react with cold water. However, if MgO is just made by burning Mg in air/oxygen, the ions in such MgO is loosely packed and such MgO would react with water to a very small extent.

Athough RaO is expected to react with water to give Ra(OH)2, no such experiment has been done due to the high radioactivity of Ra.


The oxides of all other metals not react with water.
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alkali metal oxides 同alkaline earth metal oxides 加水會變 Hydroxide, 例如:
Na2O + H2O → 2 NaOH
MgO + H2O → Mg(OH)2