Balloon in a Bottle

Students observe that air takes up space. It's only when air in the bottle escapes that more air is easily added.

Learning Objectives

  • Students learn that air takes up space and exerts pressure.

Materials

Per student:

  • 1-liter plastic bottle
  • Another 1-liter plastic bottle with a 1" diameter hole in its side that is plugged with a stopper
  • 2 balloons
  • 1 cup water

Directions

  1. Push a balloon inside a plastic bottle and stretch the balloon opening over the bottle’s top.
  2. Attempt to blow up the balloon inside the bottle. What happens?
  3. Next, place a new balloon into the second plastic bottle (the one with a 1” diameter hole in its side that has been plugged with a stopper). Stretch the balloon opening over the lip of the bottle like before.
  4. With the stopper plugging the hole, can you blow up the balloon?
  5. Unplug the stopper in the plastic bottle and attempt to blow up the balloon yet again. What happens? Why?
  6. With the balloon inflated inside the bottle, plug the bottle’s hole with the stopper. What happens to the air inside the balloon this time?
  7. Fill the inflated balloon with water while it is inside the bottle. Step outside or place the bottle over something that can catch liquid. Now unplug the stopper and watch the waterworks.

Ask yourself the following questions

  1. What happens to the balloon inside the bottle when you try to inflate it with the hole plugged and unplugged? What makes the difference?
  2. After the balloon is inflated and the hole in the bottle plugged, what prevents the air from escaping from inside the balloon?
  3. When water is placed in the inflated balloon inside the bottle, what causes it to gush out when the bottle’s unplugged?

Background

Although air is invisible, it still takes up space and is made of molecules. This is evident when the balloon is placed inside the bottle and you try to inflate it. It’s nearly impossible to add any amount of air! When the bottle with the hole is used, however, inflating the balloon is nearly effortless. The air inside the bottle is able to escape, freeing up space for the balloon to now inflate. If you then plug the hole on the outside of the bottle, the balloon will remain inflated. How does this happen? What keeps this air in place? This is a consequence of the air pressure being lowered inside the bottle when its hole is plugged. The high pressure air inside the balloon is pulled toward the low pressure area inside the bottle. When you add water inside the balloon then unplug the bottle, watch out! Unplugging the bottle will release low pressure’s hold on the higher pressure air inside the balloon and allow outside air to enter the bottle once again. Not only will the balloon collapse, but the water inside of it will also be propelled by the force of the air.