# Child's experiment to weigh air

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Baluncore
2019 Award
Fail while repeating the experiment, by balancing two identical folded garbage bags.
Almost fill one bag with air by holding open the neck while moving it through the air, (so it does not gain extra moisture from lungs, and you do not hyperventilate).
Tie off the opening, without compressing the contents.
Since buoyancy in atmosphere is 1:1 with the contained air, the weights will still balance.

We know that it is the compressed air in the balloon that is more dense than the atmosphere.

So now the fun begins. Balance two elastic balloons, Inflate one just slightly, the other to maybe twice the diameter. Which will weigh more on the balance?
To answer that question you must realise that the pressure (and so the density) of the gas in a balloon is reduced as it is inflated. Notice how hard it is to start inflating an elastic balloon, but it gets easier as the radius of curvature increases and the pressure falls. Is there some ratio where the large and small balloons balance?

Two values for a given pressure but not a single ratio of radii.

That's a cool experiment, the two balloons connected with a tube. I just fooled my wife with it. I wonder, can you pop a balloon that way or will the pressure start rising again as you near the popping radius?

When you blow up a balloon in the usual manner, you're using your exhaled breath, which is depleted in O2, and has an equivalent amount of CO2 in its place, which is denser and, indeed, weighs more than air. If you used a compressor and actually filled the balloon with "air", it would not appear to gain weight when filled, and the "experiment" would fail.
The pressure in a regular "party" balloon is only slightly above one atmosphere, and the added density due to the higher pressure is very slight.

Baluncore
2019 Award
When you blow up a balloon in the usual manner, you're using your exhaled breath, which is depleted in O2, and has an equivalent amount of CO2 in its place, which is denser and, indeed, weighs more than air.
The CO2 and O2 change is extremely small. The air you breath out is close to saturated with H2O at body temperature, some of which will condense inside the balloon. "Hot" and/or "Wet" air weighs less than dry air.

There is no question that the internal pressure will increase the density of the air in the balloon. A 1 psi internal pressure will increase the density by about ( 1 + 14.7 ) / 14.7 = 1.07 = 7%

sophiecentaur
Gold Member
The whole experiment is totally misleading for 'kids'.
We really shouldn't be taking it further without a caveat at the start of every 'PF worthy' post.

jbriggs444
Homework Helper
2019 Award
The whole experiment is totally misleading for 'kids'.
We really shouldn't be taking it further without a caveat at the start of every 'PF worthy' post.
It is not quite as bad as the Crookes radiometer which "proves" that light has negative mass.

sophiecentaur
sophiecentaur
Gold Member
Does anyone make a 'real' Crooke's Radiometer or is it just an experiment we can do in space?

jbriggs444
Homework Helper
2019 Award
Does anyone make a 'real' Crookes Radiometer or is it just an experiment we can do in space?