A general property of metal oxides is that they tend to react with water to form basic solutions of the metal hydroxide. For example BaO is a basic oxide. It dissolves in water to form barium hydroxide, Ba(OH)2. The solution contains "1" Ba2+ ion for every "2" hydroxide ions, OH1-. This means one firmula unit forms the following ions. Ba2+ OH1- OH1- where the total electric charge in the solution adds to zero.

Calcium oxide , CaO(s) , is a metal oxide. Like other metal oxides it is a basic oxide. It dissolves in water to form calcium hydroxide.

CaO(s) + H2O(l) ---->Ca(OH)2(aq)

Magnesium oxide , MgO(s), s a metal oxide. Like other metal oxides it is a basic oxide. It dissolves in water to form magnesium hydroxide, Mg(OH)2.

MgO(s) + H2O(l) ----> Mg(OH)2(aq)

Aluminum oxide , Al2O3(s) , is a metal oxide. Like other metal oxides it is a basic oxide. It dissolves in water to form aluminum hydroxide.

Al2O3(s) + 3 H2O(l) --->2 Al(OH)3(aq)

Sodium oxide , Na2O(s) , is a metal oxide. Like other metal oxides it is a basic oxide. It dissolves in water to form sodium hydroxide. This is an exothermic process that can produce so much heat that the solution may splatter and pose a threat to

Na2O(s) + H2O (l) ---> 2 NaOH(aq)