Balloon-on-a-stick
Skewering balloons without popping them; not an activity for the faint of heart.

Concepts
polymers, stress

Materials
  • Latex balloons (9-inch)
  • Bamboo cooking skewers (10-12 inches)
  • Cooking oil

Getting Ready
For this activity to work correctly the cooking skewers need to be longer than the balloon. Also, the skewers should be sharp and well-lubricated. Feel free to substitute dish soap for cooking oil.

Procedure
1. Inflate the balloon until it’s nearly full and then let about a quarter of the air out. Then tie a knot in the end of the balloon. Put the balloon aside for a moment.
2. Dip the tip of the wooden skewer into the cooking oil. The oil will serve as a lubricant.
3. At the place where you tied the balloon and on the side opposite it you will notice a thick area of rubber. This is where you will skewer the balloon. Place the sharpened tip of the skewer on the thick end of the balloon (near the place it is tied) and push the skewer into the balloon using constant, steady pressure until it goes into the balloon. Be careful not to stab yourself.
4. Push the skewer all the way through the balloon until the tip of the skewer touches the opposite end of the balloon where you’ll find the other thick portion of the balloon. Keep pushing until the skewer penetrates the rubber.
5. Make observations of the skewered balloon.
6. When ready, gently remove the skewer from the balloon. This will cause air to leak but the balloon will not pop.
7. Optional – after taking out the skewer and showing the balloons does not pop, feel free to show it can be popped by pushing the skewer into the side.
Explanation
The rubber that makes up a balloon consists of many long chains of molecules called polymers. Polymer chains are elastic and blowing up the balloon stretches them. To pierce a balloon without popping it one must choose a point where the polymer molecules are stretched out the least. Choosing a least stress point causes the molecules around the skewer to be pushed against it and subsequently keeps the air inside the balloon from rushing out. When you remove the skewer two holes are left behind where air can slowly leak out. However, there is still enough stress at the sides of the balloon to pop it if you wish.


figure13.jpg
Figure 13: A 9" latex balloon with a 12" skewer carefully placed at two low stress points.


Extension 1
Materials – latex balloon (any size)

Procedure – have students blow up a balloon, hold the nozzle closed for a few seconds and then release the air from the balloon.

Discussion – why does the balloon push the air out of itself when it is not tied?

Explanation – latex is a natural rubber and is a polymer whose chains are joined in a manner that causes them to be flexible. When the balloon is blown up the polymer chain is elongated. Upon removal of the stress the chain will fold up to its previous configuration.

Extension 2
Materials – latex balloon (any size), fortune-telling fish

Procedure – have students blow up a balloon and tie it. Next have the students take the fortune-telling fish out of the bag and lay it flat on a table. Have them hold the balloon about 30 cm above the fortune-telling fish and observe what happens (nothing should occur). Next, have one of the students should take the balloon, rub it on their hair, and then hold the balloon above the fortune-telling fish without touching it. The balloon should attract the fish and it will lift off of the table. If done properly, the fish will rise up without leaping up to the balloon.

Discussion – what causes the attraction of the fortune-telling fish to the balloon?

Explanation – when a student rubs a balloon on their head electrons are stripped off the hair and are deposited onto the surface of the balloon. Since rubber is an insulator the electrons cannot move and static electricity is present in one location. Holding the balloon above the fortune-telling fish induces a positive charge on it and the fish is attracted to the balloon. The fish starts neutral but gains a slight positive charge because of the balloon above it, and since there are opposite electric charges the two attract (opposites attract).

figure14.jpg
Figure 14: Induction causes a fortune-telling fish to be temporarily attracted to an electrically charged balloon.


*The initial activity is adapted from SteveSpanglerScience.com and Chemistry in the Toy Store by David A. Katz

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