Question #646eb

CaO is a ionic substance, that means it is made of ions, #Ca^"2+"# and oxide ions #O^"2-"#.

There are as many ionic substances insoluble in water (e.g. alumina, #Al_2O_3#, fluorite, #CaF_2#, limestone, #CaCO_3# etc.) as many others which are highly water soluble (e.g. sodium chloride, #NaCl#, calcium chloride, #CaCl_2#, magnesium sulfide, #MgS#, etc.)

In all these cases the amount of substance which dissolves in water inlets its positive and negative ions free to get hydrated by water molecules, independently from each other, given that the ions are already existing separately in the ionic lattice.

#MgO# and the other slightly soluble ionic compounds have a high reticular energy (a measure of attraction between opposite charges), a reduced hydration energy (a measure of the energy which is released when ions are hydrated in water) or both factors that contribute to lower water solubility.

About the basic (alkaline) behavior, it depends from the specific nature of the ions liberated in water. In this case, the oxide ion #O^"2-"# is a stronger base than hydroxide, and it is completely and instantaneously converted in hydroxide ion as soon as it is put in water:

#O^"2-" + H_2O -> 2OH^-#

So #MgO# is not only "dissolved" in water, but it reacts as a strong base:

#MgO(s) + H_2O(l) -> Mg(OH)_2(s)#,

where the stable compound that is slightly soluble in water is the magnesium hydroxyde. It is a strong base too (due to the #OH^-# ion), however #O^"2-"# is even stronger.