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Balloon Shish Kebab

In this activity, students discover a cool characteristic of polymers that contributes to a balloon's elasticity.

The balloon's rubber consists of many long chains of molecules called polymers, linked together like noodles stuck to each other in a plate of cooked spaghetti. These links can be stretched and compressed, giving the balloon its elasticity. If they are pulled too much, however, the balloon will break.

When a sharp skewer stick is placed through both the tie and the nubbin (end opposite of the tie), the balloon does not pop. Why Not?

The rubber at the nubbin and at the tie is looser and less fragile than around the circumference of the balloon, where the polymers are already stretched to their limit. When the skewer slides into the loose rubber, the polymers will stretch around the skewer allowing the balloon to stay inflated. A needle through the side of a balloon will cause the rubber to tear and pop easily since the polymers are already stretched.

Objectives

  • Explain how a balloon’s chemical structure gives it its elasticity.

Materials

  • Per Demo:
    2 round balloons
    sharp skewer (metal or wooden with no splinters)
    lubricant such as oil or Vaseline (optional)

Key Questions

  • What is the difference in properties between the nubbin and the side of the balloon?
  • Describe the behaviour of the molecules when the skewer pierced the nubbin and when it pierced the side of the balloon.
  • Why are the nubbin and tie ends darker in colour than the rest of the balloon?

What To Do

Preparation:

  1. Sharpen the metal skewer stick with a file. If using a wooden skewer, remove any splinters.
  2. Blow up both balloons, to about 75% full (a volunteer student could do this to show that a trick is not being played on them).

Instructions:

  1. Slowly twist the skewer through the side of the first balloon. The balloon should burst.
  2. Slowly twist the skewer through the nubbin (top) and the dark part next to the tie (bottom) of the second balloon. The balloon should not burst.

Tips: 

  • Gently twist the skewer as it goes in.
  • Coat the skewer in lubricant (e.g. wiping with an oil-dipped cloth or Vaseline) before inserting into the balloon.
  • Use a round balloon so that skewer will fit through it.
  • It may help to blow the balloon up fully and then release some air, leaving it ¾ full.

Extensions

  • What will happen if we pierce a skewer through the nubbin of the balloon and then blow it up.
  • There is a way to stick a sharp pin or skewer through the side of a balloon without popping it.
  • Put a small piece of Scotch tape on the side of the balloon and press it down well.
  • Now take the pin and press it through the tape and into the balloon. The balloon will not burst!
  • The tape sticks to the rubber in the balloon and will not allow the rubber to stretch to the breaking point when the pin pierces the balloon. In other words, the tape reinforces the cross links of the rubber polymers, and the balloon stays together.