(1) metal oxides are not necessarily basic oxides, for example, metal oxides with high valence of transition elements are mostly acidic oxides. For example: Mn2O7, CrO3, etc., because these oxides can react with alkali to generate salt and water. (2) Why must alkaline oxides be metal oxides? Let's first look at the concept of "alkaline oxides": oxides that can react with acids to generate a salt and water are called alkaline oxides. Look at salt again: it is a compound containing metal ions (or ammonia ions) and acid radical ions. When an alkaline oxide reacts with an acid, the metal ions in the generated salt can only come from the alkaline oxide (there is no metal cation in the acid), so the alkaline oxide must be a metal oxide.