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PURPOSE

Study a property of elastic polymers by passing a skewer through an inflated balloon.
 

PERFORMING THE EXPERIMENT

1. Inflate a balloon to full size (typically 9-inches in diameter), and then allow about one-third of the air to escape. 

 
2. Tie off the end of the balloon after making certain that the balloon is not longer than the bamboo skewer. 

 
3. Using a gentle, twisting motion, insert the skewer into the thicker nipple end of the balloon, the end opposite the knot.  Continue pushing and twisting the skewer until it comes out the other side of the balloon very close to the tied end.  Were you able to do this without bursting the balloon? 

 
4. Slowly pull the skewer out of the balloon.  If you place your hand over the holes, you will be able to feel the air leaking out.

 
5. Return the skewer to the table.

 
6. Place your deflated balloon in a trash container.  

 

1. Why can you sometimes ride for miles with a thumbtack through your bicycle tire and tube?  Why does air leak out of the tire and tube when you remove the thumbtack?
 

EXPLANATION

Balloons are made of a thin sheet of rubber containing many long intertwined or cross-linked strands of polymer chains.  These cross-linked polymer chains form a network of molecules.  When a balloon is stretched, the polymer network will attempt to regain its original shape, giving an elastic quality to the polymer.  This process is sometimes called "memory."  The elasticity of these polymer chains causes rubber to be stretchy.  Blowing up the balloon stretches these strands of polymer chains.  The cross-linked chains of elastic polymers in the balloon are pushed apart or separated when the skewer is inserted.  The polymer chains at the nipple end and tie end of the balloon are not stretched as tightly as those at the sides of the balloon, thus allowing the skewer to push the polymer chains apart and enter the balloon without bursting it.  A smooth, sharp skewer is used because a dull skewer is more likely to tear the rubber even at the thicker ends of the balloon.  When you remove the skewer, you feel the air leaking out through the holes where the strands were pushed apart.  Eventually the balloon deflates.  However, if  you attempt to insert the skewer through the side of the balloon, the polymer chains are already fully stretched and are unable to slide apart any farther without tearing.  Once a tear starts, it continues rapidly as the air rushes out, causing the balloon to pop.
 

EQUIPMENT NEEDED

Balloons; bamboo skewers
 

REFERENCE

Modified from Mickey Sarquis and Jerry Sarquis, Fun with Chemistry, A Guidebook of K-12 Activities, volume 1, Institute for Chemical Education, University of Wisconsin - Madison, Madison, WI, 1991, page 139.
 
 

Kids Involved Doing Science is at http://www.kids.union.edu.

Modified 10/01/02  by C. & P. Scaife