Sandie Koontz

Balloon Inside a Flask

Science Concept: Pressure. To demonstrate air pressure




I want to blow up this balloon, but also leave it untied, and have it in this flask. How am I supposed to do this?


When the water in the flask boils, it produces steam, which has gas molecules inside the flask that move very quickly, pushing out the air. When the balloon is placed over the opening, this fixes the amount of gas molecules inside as the same as outside so the pressures are equal. When the flask cools the gas molecules on the inside begin to move much more slowly and create less collisions and thus less pressure. The steam condenses into liquid water, which takes up less space than the steam (gaseous water). Since the balloon is over the flask, this changes the pressure inside the flask to less than that which is on the outside. The pressure on the outside is greater and therefore it forces the balloon inside the flask. Ultimately the gas pressure inside of the bottle, inside of the balloon, and the outside air pressures are the same. Therefore the balloon stays inflated even with the mouth of the balloon open.

Safety: Handle the flask on the hot plate with the tongs at all times.

Waste Disposal: Remove balloon from flask and dispose. Pour liquid in sink.

Reference: Summerlin, L, Ealy, J, Chemical Demonstrations, American Chemical Society, 1985,p.9.