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Balloon in a Bottle

Blowing up a balloon involves forcing additional air particles from your lungs into the balloon. These particles hit the inside walls of the balloon creating enough pressure to force the rubber of the balloon to expand and the balloon to inflate. The collision of these particles with the walls creates a high-pressure environment inside the balloon relative to the atmospheric pressure around it. This is why when a balloon is released, the high-pressure air flows out of the balloon to the low-pressure air surrounding it. "Winds blow from high to low."

When the balloon is placed inside the bottle, it will not inflate, since the bottle is already filled with air particles with no escape route. This is a great demonstration that air takes up space. The air inside the bottle compresses a little bit but not enough to permit the balloon to inflate.

However, when you punch a hole in the bottle, the air molecules in the bottle have an exit. They are pushed out as the balloon fills the space inside, resulting in room for the balloon to inflate.

If the hole is then plugged, the balloon stays inflated even when the mouth is removed. This is because the high pressure air in the balloon pushes outward harder than the low pressure air in the bottle. The air in the balloon pushes out against the walls, keeping it inflated. When the hole is unplugged, air flows back into the bottle. The air pressure in the bottle increases and collapses the balloon.

Objectives

  • Explain how air pressure works.

Materials

  • 2 Plastic drink bottles (2 litres in size, clean and dry)
    Balloons
    Scissors, thumbtack, or nail (for teacher to make the hole)

Key Questions

  • Why did one balloon inflate and the other not? If there is no change in air pressure, the balloon will not deflate (or inflate further). Why does air take up space? Why is it impossible to inflate a balloon inside a bottle? What is meant by atmospheric pressure?

What To Do

  1. ​Before the class arrives, make a hole in the bottom of one of the bottles, 2–5 milimetres across, using the scissors, thumbtack, or nail.
  2. Select two volunteers, preferably one small student and one big student.
  3. Give the bigger student the bottle without a hole and give the smaller student the bottle with the hole.
  4. Give each volunteer a balloon to place inside the neck of the bottle, stretching the open end of the balloon over the mouth of the bottle to seal it.
  5. On the count of three, challenge the volunteers to blow up their balloons!
  6. Only the smaller student’s balloon should inflate (in the bottle with the hole in the bottom).
  7. Thank the students and ask another pair of students to come up and repeat the demonstration (use new balloons).

​Teacher Tip: Kids have small lungs! The activity may be better suited as a competition between two adults.

Extensions

  • Inflate the balloon in the bottle again and cover the nail hole with your finger. Pour water into the balloon while keeping your finger over the hole. Go outside or hold the bottle over a sink before you remove your finger. Watch out for that stream of water gushing out of the bottle top!

Other Resources

Science World Resources | Full Lesson & other activities |  Air
Science World Resources | Full Lesson & other activities |  Balloons