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Fireproof Balloons

In this activity, students can defy logic by putting flame to a balloon without popping it, thanks to the ability of water to conduct heat.

Water has a high heat capacity. In other words, it takes a lot of heat and energy to change the temperature of water by 1oC.

The high heat capacity of water is due to the fact that it takes a lot of energy to separate water molecules (the physical bonds are very strong). Water has a heat capacity about four times that of air. This means that it takes about four times as much heat to raise the temperature of a balloon full of water than it would a similar sized balloon filled with air.

As the water-filled balloon is put on the flame, the heat of the flame is easily absorbed through the balloon and into the water. The water directly above the hot spot rises, cools, and sinks again, carrying away the heat from the hot spot (this cycle is called a convection current). In other words, the thin rubber surface that is being heated is cooled by the comparatively large volume of water above it. This cooling process continues until either all of the water in the balloon becomes too hot, or until a far more concentrated source of heat, such as a blowtorch, is applied to one small area on the balloon,

When an air-filled balloon is placed in a flame, it bursts. Air is a relatively poor conductor of heat away from the thin layer of rubber. As a result, the rubber overheats and the physical bonds holding the rubber polymers together are broken.

Objectives

  • Describe the relationship between water’s heat capacity and thermal heating or cooling.

  • Compare the thermal conductivity of air and water.

Materials

  • Per Demo:
    a pair of safety glasses
    2 round balloons
    matches or lighter
    a candle with candleholder
    60 ml of water

Key Questions

  • Predict what will happen when I bring the balloon with air to the flame.
  • .For the balloon with water inside, what could happen (possible answers: the balloon will burst; the balloon will take more time to burst; the balloon will take less time to burst; the balloon will never burst.)
  • Why does the balloon with water in it not burst?
  • Why is water more conductive than air (hint how are the particles arranged)?
  • What did you notice about where the flame touched the balloon?

What To Do

  1. Blow up a balloon and tie it off.
  2. Light a candle and place it in the middle of the table so the students can see.
  3. Put on your safety glasses.
  4. Hold the balloon 30–50 cm over the top of the flame and slowly move the balloon closer and closer to the flame until it pops. Note: The flame does not need to touch the balloon before the heat melts the latex and it bursts.
  5. Add about 60 ml of water to the second balloon and then blow it up to the same size as the first balloon.
  6. As before, slowly lower the balloon over the candle flame. The balloon will not pop. You can let the flame touch the balloon and it will still not pop. It will leave a sooty mark on the bottom of the balloon.

Extensions

  • How does this relate to oceans and the temperature inland vs. by a coast?
  • How would changing the amount of liquid in the balloon affect the results?
  • Boil water in a paper cup over a Bunsen burner.Make predictions regarding the heat capacity of water versus other liquids (e.g. honey or shampoo). Which will heat up faster? Do not test any flammable liquids (e.g. alcohols or lighter fluid) or liquids that produce noxious fumes.
  • How does heat travel through solids, liquids, gases, and vacuums?
  • Why is soot produced/what is soot?

Other Resources

Science World |Youtube| Demonstrate the Transfer of Heat Energy (using Hot & Cold Water)