If anhydrous barium peroxide is used in the preparation, the barium sulphate, thus obtained forms an insoluble protective coating on the surface of solid barium peroxide. This prevents the further reaction of the acid, i.e., causing the reaction to stop. If, however, hydrated barium peroxide (in the form of this paste) is used, the water causes to dislodge the insoluble BaSO4 from the surface of BaO2. BaSO4 thus settles at the bottom of the reaction vessel and the reaction continues without any difficulty.
