The Pressure-Volume Behaviour of Gas in a Balloon

Teacher’s Notes:

1. The vinyl tube and LuerLok assembly is part of the gas pressure gauge from Vernier.

2. This is not a Boyle’s law experiment. In Boyle’s law, you measure P as a function of V for a fixed quantity of gas. In this experiment P actually remains almost constant.  V increases because you are adding more gas to the balloon.   For most students, the results of this experiment are counter intuitive -- they expect the pressure to rise as they add more gas to the balloon, until the pressure becomes so great that the balloon breaks.  It is important to do a post lab discussion with them so that they can see that the pressure does not rise, because the balloon keeps stretching to accommodate the extra gas.  Make sure that they have experience with Boyle’s law as well, and that they know the reason for the difference with the results shown here.

3. There is a very sharp rise in pressure as you put the first syringe full of air into the balloon. The initial stretching of the balloon causes this. If you blow air into a balloon, you will notice this same phenomenon -- the first breath is hard, and then it is virtually no harder to blow in each breath until the balloon breaks.

4. The pressure required to expand the balloon is almost entirely due to expansion of the gas against the earth’s atmosphere, plus a small factor to stretch the rubber. Therefore, the pressure remains almost constant, independent of volume.

5. Once you have made the first initial pressure measurements for a few volume increments, you may want to increase the number of syringes of gas used in each measurement. This can significantly speed up the process. For example, you could use V = 35, 70, 105, 140, 175 mL, then fill the balloon from the syringe 5 times before making any further measurements (V = 350, 525, 750, …)

6. The balloon breaks, not because the pressure increases, but because the rubber molecules are stretched so far apart that the intermolecular forces can no longer hold them together.  In fact, the pressure is higher in the balloon during the first measurements than it is at the time when the balloon breaks.

7. As a variation on this experiment, show that P is directly related to the amount of gas in the container. To do this, attach a fixed volume flask in place of the balloon, and repeat the experiment.


Copyright © 1998 - 2008 David Dice