These 250 mL PYREX flat bottom boiling flasks have long necks with tooled mouths and take a No. 5 rubber stopper. They are manufactured from No. 7740 borosilicate glass. Wall thicknesses are carefully controlled to provide the proper balance between thermal expansion and mechanical strength for greater shock resistance. Their excellent chemical durability results in contamination-free results and long life under the toughest lab conditions. The normal working temperature for PYREX boiling flasks is 230°C.