Amonton's Law


Amonton's Law Video

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 immersed in room temperature water, the temperature of the gas increases and the pressure ireturns to the original value. The pressure differences between the gas at room temperature (~20
°C), and near the temperature of liquid nitrogen (-196°C) 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.

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 of liquid nitrogen
  • 2 beakers large enough to hold the apparatus (1 about halfway filled with water)
  • cork ring to protect the table top
  • insulated gloves




  • Set beakers on the table.  One of them should be on the cork ring, pour water in the other beaker until it's about half full.
  • Place the apparatus in the beaker on the cork ring and slowly pour liquid nitrogen over it. (It helps to have an assistant hold the apparatus upright by its handle.)  The pressure will decrease.
  • Place the apparatus in the room temperature water.  The pressure will return to close to its original value.


  • 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.