These Pyrex® round-bottom boiling flasks have long necks with tooled mouths and take rubber stoppers. 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. Excellent chemical durability for long life and contamination-free results. The normal working temperature for Pyrex boiling flasks is 230°C.