Why do metal oxides often form basic solutions in water?

Metal oxides often form basic solutions in water because they are typically ionic compounds composed of metal cations and oxide anions ($O^{2-}$).12

When these ionic metal oxides react with water, the oxide ion reacts with water molecules to produce hydroxide ions ($O{H}^{-}$), which are responsible for the basic nature of the resulting solution. For example, a group 1 metal oxide ($M_{2}O$) reacts with water according to the following equation:12

$$M_{2}O+H_{2}O\to 2MOH$$

Similarly, group 2 metal oxides ($MO$) react to form metal hydroxides:

$$MO+H_{2}O\to M(OH{)}_2$$

Because these reactions generate hydroxide ions, the resulting solutions are basic. In contrast, nonmetal oxides are generally covalent and react with water to form acids, highlighting the periodic trend where the acid-base character of oxides shifts from basic to acidic as one moves from metals on the left of the periodic table to nonmetals on the right.12