It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours a fact utilised in analysis as the"borax bead test'.ci alumina p. 150, Boric acid. H BO, properl called trioxoboric acid. may be prepared by adding excess hydro- chloric or sulphuric acid to a hot saturated solution of borax sodium heptaoxotetraborate. Na B40, when the only moderately soluble boric acid separates as white flaky crystals on cooling Boric acid is a very weak monobasic acid:it is, in fact. a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of acids, ie