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Sulfuric acid reacts with its anhydride, , to form , called "pyrosulfuric acid", "fuming sulfuric acid", "Disulfuric acid" or "oleum" or, less commonly, "Nordhausen acid". Concentrations of oleum are either expressed in terms of % (called % oleum) or as % (the amount made if were added); common concentrations are 40% oleum (109% ) and 65% oleum (114.6% ). Pure is a solid with melting point of 36 °C.