Amonton's Law

Qualitative: When liquid nitrogen is poured over the bulb of this cool, turn-of-the-century looking apparatus, a pressure decrease of the gas inside the bulb is registered on the large pressure dial. When the bulb is then held over the flame of an alcohol lamp, the temperature of the gas increases and the pressure is observed to increase. The pressure differences between the gas in the heated bulb (estimated temperature), at room temperature, and near the temperature of liquid nitrogen (estimated temperature) can be noted and plotted on a graph of Pressure vs. Temperature. 

Quantitative: Place the bulb in water baths of different temperatures and of an isopropanol/acetone dry ice bath (cold).  Record the temperature and pressure of the gas at each temperature.

The Dependence of the Pressure of a Gas on Temperature

Temperature (°C) Pressure (lbs/in2)


P1/T1 = P2/T2 or P/T = k, where k = nR/V

Construct a graph of pressure versus temperature.

Curriculum Notes 

This demonstration works well when the gas laws are being introduced. This demonstration can be performed qualitatively or quantitatively. The effectiveness of this demonstration is increased when it is accompanied by an active learning approach and by showing a computer simulation and animation of the behavior of gases at the particle level. 

Kinetic Molecular Theory Computer Simulation©2016 Gelder, Abraham, Greenbowe   Chemistry Education Resources , Oklahoma State Unversity, Oklahoma University, University of Oregon, Pearson.

The demonstration and computer simulation will help students explore all three levels of representation of Amonton's Law, i.e. following Johnstone's Triangle, and it will help make the connection to the Kinetic Molecular Theory. The qualitative demonstration takes about five minutes to perform.

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

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. 

  • 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
  • Qualitative
  • 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.
  • Quantitative
  • Place the bulb in water baths of different temperatures.  Record the temperature and pressure of the gas at each temperature.
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