# What is the weight of air? An experiment I would never do with kids.

How to mess up kids’ understanding of the world with so-called “science experiments”.

My colleague’s son recently went to a fancy-schmancy science thing. And what they learned there is “how to measure the weight of air”. So far so good.

Here is the experiment they did:

First, they measured the weight of an empty balloon. My scales are made for a different range, but let’s see what happens. My scales say “0 grams”

The weight of an empty balloon: Pretty much nothing (scales say “0”).

So here is what they did next: They blew up the balloon and weighted it again. My scales say “2 grams”.

Weight of a balloon full of air: Still not much, but more than an empty balloon. Scales say “2”.

So since a balloon full of air weighs more than the empty balloon – voila: Air has weight!

And here is what I think is wrong with this experiment*. This only works if you use a good balloon that is really hard to blow up. If you use a water ball or some other vessel that doesn’t need a high air pressure on the inside to stay inflated, it won’t work. And that is because what you measure is not the weight of air – the same volume of air was there before you inflated the balloon anyway. What you weigh is the increase in weight as you compress air. That’s why I wrote above that you need a balloon that is really hard to inflate (and my cheeks still hurts as I am typing this), so you really compress the air inside**.

Yes, you can say that since it has weight when you compress it, it likely had weight before, too. But that is not how this experiment was used, and I am pretty sure that the instructors were not aware of what exactly they were doing, but that they genuinely thought they were weighing air.

Way to mess up kids’ understanding of the world with science experiments!

How would I use this experiment if I were forced to? Firstly, I would use a beam balance and put the empty balloon on one side and the full balloon on the other. Much better than looking at tiny numbers on an electronic display. Then, I would make sure that they really understand that they are not measuring “the weight of air”, but “the weight of air under a certain pressure”. One way would be to use what we talked about above: Do the same experiment with a water ball on a beam balance – empty one on one side, inflated one on the other (now make sure you don’t have too high pressures on the inside of the ball!). Now discuss why the air in the balloon appears to weigh something whereas the one in the water ball does not.

You could even have an inflated balloon on one side of the beam and an inflated water ball on the other side (adjust for the difference in weight of the empty balloon and ball by adding small weights in a first step!). Now you would have to make sure that the volumes of the ball and balloon are the same, which is actually not an easy task. One way would be to submerge them in water and measure the difference. But then when submerging them you would probably be changing the pressure at the same time… So I’d probably go with the first option!

* full disclosure: my scales still said “0 grams” when the balloon was inflated, I gently tapped on it to get it to display a low, but still credible number… And I am pretty sure they did some cheating at the science fair, too, which makes the whole experiment even more useless.

** by the way, that is why it is so much more fun to poke a needle into a balloon than into a water ball. Unless you squeeze the water ball really hard, nothing is going to happen if it has a tiny hole in it since the air pressure on the inside and the outside are the same…

## 14 thoughts on “What is the weight of air? An experiment I would never do with kids.”

1. Mirjam

I know, right? I can see the appeal of doing something super simple as a demonstration, though, and that it would be nice to compare “air” to “no air”, even though this kinda backfired. How about a barometer inside a container which is then evacuated?

1. Ib

Du brauchst nen “like-button” für Deinen Blog. Ich lese den häufig, habe aber oft nix vernünftiges hinzuzufügen da Dein Text schon so ausfüllend (und verständlich!) ist. .-)

2. David Smith

Is there more air inside the balloon once you’ve blown it up?
Yes, as you had to put more air inside it to raise the pressure i.e. blow into it.
Does the balloon plus air inside it weigh more than the balloon without (or very little) air in it?
Yes
Therefore the air has weight. Whether it has been compressed or not makes no difference.

1. Mirjam

But the question was what the weight of air is, not whether or not air has weight. Which, I’m sure you’ll agree, depends on both volume and pressure.

1. Robert Bowers

Look at atomic weights of the gases making up said air, add them by volume and weight is increased.

1. Robert Bowers

Without there would be no pressure, therefore air has mass and takes up space! Limited space allows for increased pressure of air. Pressure inside the balloon causes the density of the air to be increased.

2. Peter

What you are forgetting is that there is a buoyant force on a ballon full of air, which by archimedes is equal to the weight of air that the balloon displaces, so Mirjam is exactly right. The difference between the weight of the balloon and the buoyant force is due to a) the plastic skin, and b) the fact that the air inside the balloon is denser because it has been compressed.

3. will caro

A balloon with air in it must weigh more that an empty one because the ‘squishing’ caused by the elastic skin is causing more air to be crammed into the space. If the balloon is hard to fill up then there is more pressure and therefore more air than there would be in the same volume if uncompressed. You wouldn’t expect a paper bag to weigh more opened up rather than flat, would you?
But can the small increase in the weight of the balloon be measured easily? I agree, a beam balance is by far the most visual.
It is NEVER right to cheat an experiment – even primary school kids can catch you out.
Science shows are just that – a script. They have very little real content and even less understanding of what is going on. They rely on the kids NOT asking questions, and that ain’t teaching!
Will

4. Mel

There is nothing wrong with the experiment procedure BUT the purpose is incorrect.

This experiment should be used to demonstrate to kids that GASES TAKE UP SPACE AND HAVE MASS (sorry for the caps but I can’t bold on here). The right terminology and purpose needs to be made clear to students for an experiment to help them develop correct scientific conceptual knowledge