A black plastic tube like bag filled with air rises more than 10 feet into the air!

Why is it floating? An Introduction to the Gas Laws

This outdoor activity is a great way to introduce the study of the Gas Laws!

Filling a black plastic trash bag with air or using a “Solar Tube” (an extra-long black plastic tube available from many science supply companies), gives students the opportunity to study some of the properties of gases.  

In doing this activity, students will notice that gases expand when heated and that hot air is less dense than cool air.  The Gas law illustrated is Charles’ Law: Volume and temperature are directly related; as one increases, the other decreases.


solar tube
Solar tube


This activity works best on a cool but sunny day with no wind.


The student will:

  1. Open the bag or tube and fill it with cool air in the shade near ground level.  The best way to do this may be for a couple of students to hold open one end of the bag or tube and run with it.  If there is not a good space to do this outside, fill the bag indoors using a fan, then go outside.
  2. Quickly tie off the open end or ends with the kite string, when the bag or tube is fully expanded.
  3. Then tie the reel of kite string to one end.  When using the long tube it works well to have a reel of kite string on each end.
  4. Move into direct sunlight.  Place your hands on the bag to feel it becoming rigid as the air inside it expands from the absorbed heat of the sun.  
  5. As you feel the bag begin to rise, quickly record the temperature of the air and the temperature of the surface of the bag or tube.  (If you notice that the bag is not inflating it may have a hole in it.  Tactually locate where air is being released and patch the hole with the packing tape.)
  6. Take turns holding on the bag or tube. There will be significant pull as it rises upward.  (Do not release the bag, as it will continue to rise and could become a hazard to airplanes!) 

Reel in the bag at the end of the class.  If it has not ripped, it can be reused.

solar tube
Solar tube


Additional information needed to solve Gas Law problems:

Remember that gas volumes are proportional to temperature only when temperature is expressed in Kelvins.  The kelvin is the SI unit of temperature in which each unit is referred to as a kelvin not a degree.  Absolute zero, 0 K, is minus 273 degrees Celsius (-273 degrees C.).

NGSS Standards:


By Kate Fraser

Collage of Why Is it Floating?

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