Exploring Heat & Pressure

If you quickly pop off the top of a soda pop bottle, you will see a cloud form in the neck of the bottle!

Materials

• A cold bottle of soda pop with a pop-off cap or screw off lid.
  

• A bottle opener (if you are using a glass bottle with a pop-off cap).1 glass jar, empty (top opening about 5 cm)

• A flashlight can be used to illuminate the cloud in the bottle. newsprint paper

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Procedure:

• Use a cold bottle of soda pop and quickly remove the top, releasing the pressure inside.
  
  
• Observe the neck of the bottle, above the liquid, for any changes that may occur.
  
  
• If you like, have a partner shine a flashlight into the neck of the bottle as you open it - this may help you observe the condensation in the bottle.

Leading the Activity:
Explorations, Questions, and Sharing

• How cold does the bottle need to be in order for the "cloud" to form? Try a bottle that is cool to the touch, but not cold. How about a bottle that is at room temperature?
  
  
• Which works better, a pop-off cap or a twist-off cap? Why do you think that is?
  
  
• How about a sealed bottle of some other beverage? Try a non-carbonated beverage and see what happens.
  
  
• Can you explain your observations using carbonated and non-carbonated beverages? Does the beverage need to be carbonated? Why or why not?

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Background: What's going on?

Pressure and temperature are directly related. Changing the pressure in a system alters the temperature. If the pressure is released, the temperature will drop and the opposite is true for increasing the pressure. This works both ways. Changing the temperature (adding or removing heat) changes the pressure in the system.

When the top of the bottle is removed, the pressure in the bottle drops very, very rapidly. You can often hear the release of pressure even if you do not notice the "cloud." As the pressure is released, the temperature drops dramatically - often low enough for the water vapor in the neck of the bottle to condense into a foggy mass.

 

Credit where credit belongs...

This activity is described in greater detail, along with a great deal of insight, wit, and humor in Craig F. Bohren's book, Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics (John Wiley & Sons, Inc., 1987). Thanks for the great activities in your book, Mr. Bohren!