pair. Lewis's proposal gives an explanation to the Brønsted–Lowry classification in terms of electronic structure. HA + B <=> A- + BH+ In this representation both the base, B, and the conjugate base, A−, are shown carrying a lone pair of electrons and the proton, which is a Lewis acid, is transferred between them. Lewis later wrote in "To restrict the group of acids to those substances that contain hydrogen interferes as seriously with the systematic understanding of chemistry as would the restriction of the term oxidizing agent to substances containing oxygen." In Lewis theory an acid, A, and a base, B, form an adduct, AB, in which the electron pair is used to form a dative covalent bond between A and B. This is illustrated with the formation of the adduct H3N−BF3 from ammonia and boron trifluoride, a reaction that cannot occur in aqueous solution because boron trifluoride reacts violently with water in a hydrolysis reaction. BF3 + 3H2O -> B (OH)3 + 3HF HF <=> H+ + F- These reactions illustrate that BF3 is an acid in both Lewis and Brønsted–Lowry classifications and emphasizes the consistency between both theories. Boric acid is recognized as a Lewis acid by virtue of the reaction B(OH)3 + H2O <=> B(OH)4- + H+ In this case the acid does not dissociate; it is the base, H2O that dissociates. A solution of B(OH)3 is acidic because hydrogen ions are liberated in this reaction. There is strong evidence that dilute aqueous solutions of ammonia contain negligible amounts of the ammonium ion H2O + NH3 -> OH- + NH+4 and that, when dissolved in water, ammonia functions as a Lewis base. Comparison with the Lux–Flood theory The reactions between oxides in the solid or liquid state are not included in Brønsted–Lowry theory. For example, the reaction 2MgO + SiO2 -> Mg2 SiO4 does not fall within the scope of 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. 2H+ + MgO(s) -> Mg^{2+}(aq) + H2O Dissolved SiO2 has been predicted to be a weak acid in the Brønsted–Lowry sense. SiO2(s) + 2H2O <=> Si(OH)4 (solution) Si(OH)4 <=> Si(OH)3O- + H+ According to the Lux–Flood theory, compounds such as MgO and SiO2 in the solid state may be classified as acids or bases. For example, the mineral olivine