Air Pressure and the Mass of Air

Plastic bottles before processing
photo source Wikipedia

It began with me telling them that I had read somewhere that the same amount of plastic went into the hard, large plastic test tubes, the small bottles of juice (the kind with the same tops as the 2 liters) and the regular 2-liter soda bottles. The larger the bottle size, the more the plastic is blown out, making it thinner. Then James said that if the two bottles were put on a balance scale then, they would balance out. This lead to our testing out his hypothesis.

We used a pole and found the spot at which it balanced. We put a piece of tape at this point so we could always find it. We then tied two pieces of 6 inch string to the pole, four inches in on each side. We confirmed that it still balanced at the same spot. Then we tied to the other end of the strings two of the different sized bottles.

We then noted that the pole still balanced.

Then we took the bottles off and tied two identical balloons on the end of the poles instead. The balanced, of course.

Then I blew one of the balloons up. The pole was was very out of balance, leaning to the side of the blown-up balloon, thus proving that air does have mass.

Why then did the 2-liter bottle, which contained more air than the smaller bottle, balance with the smaller bottle in the first experiment?

We discussed  that the difference was the fact that with the 2-liter bottle no air had been forced into it and that the air on the outside of the bottles would evenly balance out the air on the inside of the bottle, not causing any mass (or weight) from the air.

source: Exploring Creation with Chemistry, Jay Wile