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  Experiment #2

  For this second experiment we will try to stick a sharp pin through the side of a balloon without popping it.

  You will need:

  • A balloon
  • A sharp pin
  • Sticky tape

  Cut a small piece of tape and stick it to the side of the balloon. Make sure you press it down well. Now press the pin through the tape and into the balloon.

  If you do this carefully, the balloon doesn’t burst: because the tape sticks to the rubber in the balloon, it doesn’t allow the rubber to stretch to breaking point when the pin pierces through. So the cross-links are reinforced thanks to the tape, and the balloon stays inflated.

Putting a Needle Through a Balloon

We all know that you can pop a balloon with a needle. But how can you put a needle through it without popping it?

These two kitchen experiments are not just cool party tricks — they show some interesting science too.

Experiment #1

You will need:

• Some balloons
• Two knitting needles or long wooden or metal skewers
• A jar of Vaseline

Blow up two balloons, but not too full, and tie them shut. Take the first balloon. Dip the tip of a knitting needle (or skewer) in the Vaseline, and spread the Vaseline along the entire length of the knitting needle. If you are really careful, you can now push the knitting needle all the way through the balloon without popping it.

Try to be patient and keep pushing the knitting needle with a gentle twisting motion into the end of the balloon directly opposite the knot. Push and twist the knitting needle until its tip emerges.

Now try to stick the knitting needle into the side of the second balloon. What happens? The second balloon bursts. But if you were careful, the first balloon didn't pop. Why?

The rubber in a balloon consists of many long molecules that are linked together. Think of them as being a bit like a plate of spaghetti, where the strands stick together.

The scientific name for these long molecules is “polymers”. When molecules of a polymer are chemically attached to each other, this is called cross-linking.

Thanks to these cross-links the polymers are held together, and they can even stretch — up to a point. But once the force pulling on the cross-links is too great, the links break and the polymer pulls apart.

Look carefully at the rubber near the ends of the balloon where you first inserted the knitting needle. See whether it looks lighter or darker than the rubber in the rest of the balloon. This rubber at the end of the balloon is stretched out less than in the middle of the balloon. So there is less force pulling on it.

The tip of the knitting needle breaks some of the cross-links, pushes aside the molecules of rubber, and slides into the balloon. But enough cross-links still remain to hold the balloon together so it doesn’t burst.

But there are fewer polymer molecules at the side of the balloon. So when you push the knitting needle through the rubber in the side of the balloon, it breaks a few of the cross-links, but the tension on the remaining cross-links is too great and the balloon bursts.