• Version 1
  • A balloon on an empty flask expands when the flask is heated.
  • Don't heat flask too long.
  • A fire extinguisher is available in the cart.






         (use 1/2 m clamping stand and 1 L flask)
         (position flask above flame to protect balloon)
         (check fire extinguisher)
         (close gas valve and allow tubing to drain before transport)


  • When the flask is cooled in LN, the balloon is drawn inside. You will have to help it slide into the neck to keep it from tearing.
  • Raise and lower the flask on the stand.

  • Version 2
  • Boil a small amount of water in a flask.
  • When steam is streaming out of the opening, turn the bunsen burner off.
  • A fire extinguisher is available in the cart.






         (use 1/2 m clamping stand and 1 L flask)
         (prestretch balloon)
         (check fire extinguisher)
         (close gas valve and allow tubing to drain before transport)

  • Put a balloon over the opening as soon as boiling stops.
  • The balloon will be drawn into the tube.  You will have to help it slide into the neck to keep it from tearing.
  • The inversion happens suddenly, but it takes several minutes for the balloon to fill the flask.  Immersing the flask in a bowl of water speeds the process.
  • As a variation, start with the balloon in the flask and ask students how it got that way.








        (use bowl of water on lab jack to cool flask)



  • Reheat flask to reinflate balloon, or carefully lift lip of balloon to break seal and pull it out.
  • Do not heat flask too long or balloon will burst.  Remove balloon when pressure is minimal.

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