The oxides can be acidic, basic or amphoteric
Na2O, MgO basic, make hydroxides (14,9)
Al2O3 SiO2, are insoluble – no pH value (7) but we use limestone to remove SiO2 as an acid impurity in steelmaking Al2O3 is amphoteric – can neutralise acids and alkalis
P4O10, SO2 (&SO3) acidic (0,3,(0))
Because: SO2 moderately solubleweak, SO3 very solublestrong
Alkali Acid across period.