Amonton's Law

When liquid nitrogen is poured over the bulb of this cool, turn-of-the-century looking apparatus, a pressure decrease is registered on the large dial. When the bulb is then held over the flame of an alcohol lamp the pressure is observed to rise. 

Curriculum Notes 

This demo is great for use when the gas laws are being introduced. This demo takes about five minutes to perform.

Lead Time 
One day of lead time is required for this project.
Discussion 

Amonton's Law states that the pressure of an ideal gas varies directly with the absolute temperature when the volume of the sample is held constant. P1/T1 = P2/T2 or P/T = k, where k = nR/V. Though the volume of the bulb of the apparatus does change slightly as the metal contracts and expands in response to the temperature changes that it is subjected to, it does not change enough to account for the change in pressure that is observed. 

Materials 
  • Amonton's Law apparatus
  • large Dewar flask containing a liter or two of liquid nitrogen
  • metal tray to catch liquid nitrogen
  • cork rings to set tray on to protect the table top
  • alcohol lamp
  • butane lighter
  • insulated gloves
Procedure 
  • Set tray on cork rings on table.
  • Place the bulb of the apparatus in the tray and pour liquid nitrogen over it. A rapid decrease in pressure will be observed.
  • Remove the bulb from the liquid nitrogen and observe it for a minute or so. The pressure should begin to rise.
  • Light the alcohol lamp.
  • Heat the bulb of the apparatus over the flame. Heat it gently until the pressure begins to approach atmospheric, then it can be heated a bit more vigorously. The pressure should be observed to increase.
  • Do not heat to maximum pressure.
Safety Precautions 
  • Liquid nitrogen is extremely cold. There is a risk of frostbite if it comes in contact with your skin. Be careful not to allow it to come into contact with your skin. Be particularly careful of the liquid nitrogen as it splashes off of the bulb. It can spill off the table onto your feet and legs. Wear goggles.

  • As with any demo using flame, keep the area clear of combustible materials and keep a fire extinguisher near at hand.
  • The bulb will be hot when this demo is done. Do not touch it until it cools. It could burn you.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group