Hands-on Science Carnival 2010 Activity Stations: Material Properties

Balloon-ka-Bob

[Shopping List: 12” latex balloons; Shish-ka-bob skewers; baby oil; balloon pumps]

1. Inflate a 12” balloon to only 8-9” in diameter and tie it off.
2. Dip the tip of a skewer in baby oil.
3. Starting at either the top center of the balloon, or near the neck (i.e. the areas where the latex is not stretched as much, indicated by a slightly darker color), rotate the skewer back and forth as you gently push it into the latex. If you’re careful it should poke in without popping the balloon.
4. Push the skewer all the way into the balloon until it touches the opposite side, then repeat the procedure to poke the skewer out again.

What’s Happening: Latex rubber is a very long chain-like molecule called a polymer that is cross-linked to many other molecules to form sort of a big spider web structure which can stretch back and forth, making the gaps between the molecules larger or smaller. This is called elasticity, and rubber is a very elastic material, which is why it can stretch a lot and still spring back. Normally the gaps between molecules are very small, so small that even air molecules can’t get through them. If you’re very careful however, you can push apart some of the molecules with the very sharp tip of the skewer and poke it in between them, while the elasticity of the rubber still seals it tightly against the sides of the skewer and prevents any air from escaping. The baby oil lubricates the point and sides so it slides in easier, and also helps improve the sealing. If you poke too hard you might break some of the molecules, which creates a small tear in the balloon, then the air pressure inside quickly enlarges the tear and pops the balloon.