Why is it floating? An Introduction to the Gas Laws

By Kate Fraser on Aug 14, 2015

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.


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


  • Black plastic trash bag or solar tube
  • Kite string
  • Scissors
  • Cellophane packing tape (for repairs of ripped plastic)
  • Talking thermometers

solar tube


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


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.).


Chemistry: Concepts and Applications, Glencoe Science, 2009, pages 348-350 and 391-393. www.chemistryca.com    http://www.teachersource.com/category/s?keyword=slr-222+solar+tube This is one of the companies that sell solar tubes that can be used in this activity.

NGSS Standards:

  • PS3.A: Definitions of Energy  -  Energy is a quantitative property of a system that depends on the motion and interactions of matter and radiation within that system. That there is a single quantity called energy is due to the fact that a system’s total energy is conserved, even as, within the system, energy is continually transferred from one object to another and between its various possible forms. (HS-PS3-1),(HS-PS3-2)
  • At the macroscopic scale, energy manifests itself in multiple ways, such as in motion, sound, light, and thermal energy. (HS-PS3-2) (HS-PS3-3)
  • These relationships are better understood at the microscopic scale, at which all of the different manifestations of energy can be modeled as a combination of energy associated with the motion of particles and energy associated with the configuration (relative position of the particles). In some cases the relative position energy can be thought of as stored in fields (which mediate interactions between particles). This last concept includes radiation, a phenomenon in which energy stored in fields moves across space. (HS-PS3-2)

gas laws collage

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