The reactions between oxides in the solid or liquid state are excluded in Brønsted–Lowry theory. For example, the reaction 
is not covered in the Brønsted–Lowry definition of acids and bases. On the other hand, magnesium oxide acts as a base when it reacts with an aqueous solution of an acid.
Dissolved SiO,silicon dioxide has been predicted to be a weak acid in the Brønsted–Lowry sense.
According to the Lux–Flood theory, oxides like MgO and SiO in the solid state may be called acids or bases. For example, the mineral olivine may be known as a compound of a basic oxide, MgO, and silicon dioxide, SiO, as an acidic oxide. This is important in geochemistry.