Balloon in a Flask (Demonstration)

Pressure   :    Vacuum

Demonstrates gas laws by allowing students to apply knowledge to an unexplained curiosity

Safety goggles and heat resistant gloves should be worn by demonstrator

Large, good quality balloon
500 mL Florence flask or 250 mL Erlenmeyer flask (long neck pop bottles work also)
Hot plate or bunsen burner
Oven mit or hot pad
10 mL water

1. Pour about 10 mL water into the flask.
2. Heat the water to boiling (vigorously).
3. Allow most of the water to turn to steam, driving out the air.
4. Remove the flask from the heat.
5. Quickly lower the body of the balloon into the flask and stretch the neck out to seal the top of the flask.
6. Allow to cool.
7. To reverse the process, reheat the flask.  The balloon will “crawl” out of the flask and expand itself over the top of the flask.

As the water vapor cools, it condenses to water.  As the pressure in the flask drops, the balloon expands to equalize pressures, inside and out.  The effect is that the balloon very nearly lines the flask.

Nothing special

Ginger Chateauneuf, 2000.