PURPOSE: To demonstrate surface tension in a counterintuitive way.

DESCRIPTION: Use two identical balloons. Blow up one balloon on the tube and clamp it. Then blow up the other balloon to a different size and slip it onto the other end of the tube. Q: When you remove the clamp, what will happen?: (a) the small balloon will get smaller and the large one larger, (b) the two balloons will become equal, or (c) they will stay the way they are. A: The small balloon will blow up the larger one, and get smaller, due to surface tension effects. The rubber is thicker in a smaller balloon, and thus produces greater surface tension. Click your mouse on the photograph above to view an mpeg video of the action.

SUGGESTIONS: Let the students vote before doing the experiment. Hold the smaller balloon on top as you release the clamp; then suggest that gravity pulls the air down into the lower balloon.

See Question of the Week #5 for information on using this demonstration to enhance class involvement.

Click here to see an excellent video showing surface tension in a water balloon when the balloon is punctured by a pin. Notice how the rubber balloon shrinks and disappears, leaving the water filling the balloon still intact. (Soure unknown.)

REFERENCES: (PIRA 2A10.51) See Demonstration Reference File for relevant papers. See also F3-03: SURFACE TENSION - SOAP BUBBLES.

EQUIPMENT: Two 12" balloons with connecting tube and clamp.

SETUP TIME: None.