Question

are all metal oxides acidic..?

Answer 1

Well, what makes an acid?

In order for a metallic oxide to be acidic, it would have to decrease the pH of neutral water to anything below a pH of 7. Take for example sodium oxide Na2O, it does not carry any hydrogen to the solution. When added to water, the total ionic equation is:
\[Na(_2)O + H(_2)O --> 2Na(^+) + O(^_2) + H(^+) + OH(^-) \]
If anything the solution might become more basic as the now free oxide ion connects with any loose hydride ions.