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Consider the experiment in this link.

The experiment includes using a ruler as a lever, with an inflated balloon on one side and a balloon which is not inflated on the other.

The aim of the experiment is to show that air has mass.

I have seen many kids performing similar experiments.

But, if the air pressure inside the balloon is equal to that outside, then the buoyant force will cancel out the weight of the air inside the balloon, won't it?

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How can an experiment be wrong? If your experiment does not match the theory, it's the theory that's wrong, not the experiment. –  ACuriousMind Jul 28 at 17:44
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@ACuriousMind Alright, i edited the title. –  Pkwssis Jul 28 at 17:46
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But why do you think that the air pressure inside the balloon is equal to the outside? Aren't you doing work when you blow up a balloon? –  Peter Shor Jul 28 at 17:49
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@Pkwssis Please repeat the essentials of the experiment in your post, as the link can die at some point, which makes your question pointless. We would like to avoid that. –  Bernhard Jul 28 at 17:52
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I have the feeling that what you say is that this experiment is a bit misleading. Of course air has mass. But if the balloon was filled with air at ambiant pressure, it won't make the ruler move... It is because the air inside the balloon is denser that outside, and thus weights more. –  Martigan Jul 28 at 17:53

5 Answers 5

up vote 47 down vote accepted

I can think of at least four things going on in this experiment that need pointing out:

  1. When you inflate a balloon by mouth, the air is warm: this makes the air inside the inflated balloon slightly lighter than the air it displaced
  2. The air inside the balloon has 100% relative humidity at 37C, and condensation will quickly form on the inside of the balloon as the air inside cools down.
  3. The air inside the balloon contains carbon dioxide, which has higher density than room air (molecular mass of 12+16+16 = 44 amu, vs oxygen at 32 amu and nitrogen at 28 amu - ignoring small isotopic effects, and ignoring Argon).
  4. The pressure inside the balloon is larger than outside - this increases the density

So how large are each of these effects?

  1. Warm air: 37C vs 20C results in drop in density of 0.945x (293 / 310) or -5.5%
  2. Moisture: partial pressure of water at 37C is 47.1 mm Hg source which is about 0.061 atmospheres. Assuming that pressure is constant, this water (mass 18 amu) displaces air (mean mass 29 amu), so the density of the air decreases by 0.061 * (29 - 18) / 29 = 2.3%. If we allow the air outside the balloon to have 60% relative humidity (with saturated vapor pressure of 10.5 mm Hg), it would be slightly less dense than dry air (10.5*0.6/760*(29-18)/29 = 0.3%) making the net difference -2.0%. Note that much of this moisture will condense when the balloon cools down - little droplets will form on the inside of the balloon. With the air inside still saturated, its density will be 0.1% lower than on the outside; the net result amounts to 2.9% of the mass of the air in the balloon.
  3. Carbon dioxide: the exhaled air has 4 - 5 % carbon dioxide source: wikipedia, with an equivalent drop in oxygen. The density of exhaled air is therefore higher than that of inhaled air by 0.045 * (44 - 32) / 29 = +1.9%
  4. Pressure in the balloon: from this youtube video - time point 3:43 I estimate the pressure increase in the balloon at 23 mm Hg, resulting in an increase in density of 2.9%

Summarizing in a table:

factor      effect   at room T
temperature -5.5%       0.0%
moisture    -2.0%       2.9%
CO2          1.9%       1.9%
pressure     2.9%       2.9%
net         -2.7%       7.7%

A freshly inflated balloon will thus have only a slightly lower density than the air it displaced, because the temperature + moisture effect is greater than the other two. After you wait a little while, the temperature will equalize and the density of the air inside the balloon will be greater - by 7.7%, with more than half of that not caused by the pressure in the balloon...

In summary: the experiment described in your link measures the difference in density between air in a balloon, and ambient air. Since the density of the air inside the balloon is higher than the density outside the balloon, one may conclude that the air inside the balloon has finite density. One may NOT conclude that the medium outside the balloon (which we believe to be "dry air") has any density at all - since nothing in this measurement tells us about the air outside the balloon.

If you did the experiment carefully with a balloon initially filled with warm air, and you allowed the air to cool down, you might be able to tell that the balance shifts - in other words, that there must be a change in the buoyancy experienced by the balloon as it cools down. THAT would be an experiment to demonstrate "air has mass" (volume of balloon decreases, and it experiences less buoyancy). From the experiment as described (popping the balloon), we learn that "exhaled air has mass". That is not the same thing.

If you used an air pump (balloon pump) to inflate the balloon, the first three components would go away and you are left with the difference due to the pressure only - 2.9% of the mass of the air in the balloon.

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I am not sure the temperature really affects, I think it has enough time to cool down; the rubber has a much higher specific heat, so there is no need even to go through. I have been inflating balloons today, and they didn't feel warm (but sadly didn't have my IR thermometer nearby). Other than that, it is a very good analysis of all the factors! –  Davidmh Jul 28 at 20:42
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@Davidmh I would not expect a balloon to feel warm given the thermal conductivity of the rubber and the relative heat capacity of the air. To test whether the air inside the balloon is significantly warmer than the environment, I would see whether the balloon diameter changes just after inflating (when volume drops 4.5%, the diameter should drop 1.5%, or about 3 mm for a 20 cm balloon. Need quite a careful setup to measure it.) –  Floris Jul 28 at 20:47
    
Since "air" isn't very clearly defined in the experiment, the variations in components doesn't seem particularly important, though water vapor from breath might be one exception. Regardless, assuming absence of "mass", would "density" of any normal component be relevant? I.e., does the relative density of CO2 matter at all (if it had no mass)? For the degree of rigor of the experiment, how could one successfully separate "density" and "mass"? –  user2338816 Jul 29 at 5:53
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For these reasons the experiment could be greatly improved using a balloon pump so that you can fill the balloon with "dry air" (or to be precise, whatever air is already around). That way you directly prove that air has mass, as opposed to proving "whatever I blow out into a balloon" has mass. Of course whatever you're exhaling is a component of the air in the room, but it doesn't seem trivial to demonstrate that you haven't spat/dribbled into the balloon while inflating it, so best to eliminate the possibility. –  Steve Jessop Jul 29 at 9:36
    
@stevjessop an air pump would definitely improve the rigor of the experiment. Good suggestion. –  Floris Jul 29 at 14:26

That's a perfectly good way to show that the density of air increases with pressure, and therefore that air must have a mass.

When the scales tilt down on the side of the unburst balloon it shows the volume contained within the balloon has a higher mass than the same volume of air at atmospheric pressure. This means the density must be greater. we don't know the pressure inside the balloon, though we know it must be higher than atmospheric pressure because the elastic skin of the balloon is exerting a force on the air inside it.

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Yes..correct..i didn't consider the force due to the elastic skin.. –  Pkwssis Jul 28 at 17:57
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The best way I've shown that the density of air increases with pressure is to use two balloons. Take one and insert an empty balloon inside it before inflation. The second balloon, insert a second balloon as well, but blow up the inside one so the pressure is double! When both are dropped at the same time, the second balloon should fall a little faster. I have not tried this since I was very little, so if it does not work forgive me. –  Phil Jul 29 at 19:34
    
While the reasoning as you point it out is valid, I'd still be afraid that children who paid less than perfect attention might miss the pressure argument, and might instead think they measured the air in the baloon without taking displacement into account. That interpretation looks so obvious at first glance… So I'd still not consider this a “perfectly good way”. –  MvG Jul 30 at 9:03

Sometimes it's helpful to explain the rationale of experimental design to a student as testing "competing alternative hypotheses."

A positive result by this experiment would be strong evidence against the alternative hypothesis that the air blown into the balloon has no mass.

A negative result however would not prove that the air has no mass. Further experimentation would be necessary to understand why the unexpected result was obtained.

Aside, perhaps the best way to do this is to follow the experimental approach of Lavoisier - and demonstrate the conservation of mass. Weigh a solid compound, heat the solid to drive off a gas (capture the gas) and then measure the weight lost by the solid and (less easily) the weight of the gas. Increasingly better experimental technique should show quantitatively better agreement with conservation of mass.

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John Rennie's answer correctly points out that the correct chain of reasoning here is a bit complex:

  • On one side of the balance we have a column of air going up to space, a balloon, and some compressed air, and have displaced an amount of the air column equal to the volume of the compressed air.

  • On the other side we have a similar column of air and a baloon.

  • The first side dips, and so has more mass.

  • Therefore air is compressible into a form that has greater density than the mass of air displaced.

  • Therefore air has mass.

This complex chain of reasoning is likely to be lost on all but the brightest students. And a student could easily point out that a balloon containing a mixture of air and helium could be produced so as to cause the other side of the balance to dip, but from this we would not conclude that air-plus-helium is massless.

A better experiment would be to have two identical rigid bottles, evacutate the air from one, and show that the evacuated bottle has less mass, either by balancing them against each other or by measuring their inertia. That of course has the down side of requiring specialized equipment.

An experiment that I like that shows that air has mass is as follows:

  • Get a helium balloon, a bowling ball and a station wagon.
  • Put the balloon in the front seat and the bowling ball in the back.
  • Get up to speed and keep the objects stationary with respect to the car.
  • Hit the brakes.

The bowling ball rolls forwards, indicating that it has mass. It has inertia, therefore it has mass. The helium balloon floats backwards. What causes the balloon to go backwards? The air pressure must have increased at the front of the car and decreased at the back when the brakes were applied. Some of the air from the back of the car kept going forwards when the brakes were applied, therefore the air, like the bowling ball, has inertia, and therefore mass.

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Eric - as you point out, "this is hard", and most experiments would require specialized equipment. I think only the hot air / cold air experiment (last paragraph of my answer) can be done in a "kid setting" - but even then, the difference in lift is quite small and would still need a careful experimental setup (no drafts...) –  Floris Jul 28 at 23:02
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"from this we would not conclude that air-plus-helium is massless" - although we might conclude that helium contains a high component of phlogiston. By which I mean that these experiments only "prove" anything with the support of a surrounding theory. The theory itself has been developed and "proved" (i.e. not disproved) by very many experiments over a long time. It's often difficult to detect when you're being hoodwinked by a simple experimental proof of something, where the assumed theory is actually harder to believe than the single thing being demonstrated. –  Steve Jessop Jul 29 at 9:42
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@SteveJessop: Of course you want to dephlogisticate the air before the experiment; it goes without saying. :-) –  Eric Lippert Jul 29 at 13:01
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The default position is air has no mass and helium has negative mass. –  Joshua Jul 29 at 19:14
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If you argue using inertia, you might as well avoid the station wagon: simply blow up the baloon and release it without tying a knot. You'll have a little baloon rocket, and since the total inertia has to stay the same, the air must have inertia opposite to that of the baloon, so it must have mass. –  MvG Jul 30 at 9:06

  While the experiment you describe is legitimate, it has a complexity that could make it difficult to understand ("oh, my breath has mass!").

  The simpler way to do this is to put a container (I've seen a basketball used, but balloons work as well) on a scale or a balance with regular weights on the other side. Use a bicycle pump to add air to your container, and see the mass of the container increase.
  It's difficult to argue that the pump is importing anything other than the same air that surrounds your container, and the effect is clear. This worked well for me in my youth.

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