The Borosilicate Glass Flat Bottom Boiling Flask is a 250mL, long-necked flask for heating and boiling liquids. The neck accepts a #5 stopper (sold separately) and has a tooled mouth for a smooth stopper fit. Flat-bottom flasks can be set on a surface. This flask is made of No. 7740 clear borosilicate glass for durability and chemical- and temperature-resistance, and its uniform wall thickness provides mechanical strength for shock-resistance. Borosilicate glass has a low coefficient of thermal expansion and is resistant to thermal stress. Boiling flasks provide uniform heating of liquids for quality testing, sampling, and research in chemistry, education, research, and life science applications.