Where Did the Gas Go?

After a couple of hours, you may observe that the CO₂-filled balloon is noticeably smaller than the air-filled balloon. By the next day, there won't be any doubt. Where is the gas going?

"Aaahhh," you might say, "I remember talking about effusion, which explained why helium-filled balloons deflate faster than air-filled ones. Tiny helium atoms can escape faster through itty-bitty holes in the balloon than the molecules in air can. It must be the same effect!" That's a very good connection to draw, so let's see if it fits with what we know about the sizes of gas molecules.

Here are some gas molecules, drawn more or less to scale:

Most of air is nitrogen, which exists as N₂ (shown in blue). Oxygen is O₂ (red) and carbon dioxide is CO₂ (gray and red). Other gases shown for comparison are argon (aqua), water (red and black), and methane or natural gas (gray and black). Helium (He) is the little orange circle!
Does it still make sense to say that He and CO₂ effuse quickly because they are so much smaller than the other molecules in air? Of course not! CO₂ is actually the biggest molecule shown! What is going on?

I have to admit that I was puzzled by this at first, because I was thinking just like you are. But it turns out that the speed with which gas molecules escape from a thin-walled container like a balloon isn't just determined by the size. And carbon dioxide is known to move through many kinds of rubber very quickly indeed (rubber is said to be very permeable to CO₂).

But why? It turns out that CO₂ is very comfortable mixing with the rubber. So these molecules get absorbed into the balloon wall, and work their way through to the outside (a process called diffusion). Other molecules, such as nitrogen and oxygen, don't mix so easily, and they stay in the balloon longer, although they eventually diffuse out as well.

So, the moral of this story is, you can't always assume that one scientific principle applies in every case - sometimes you have to look deeper. And frankly, that's one of the things that makes science so interesting!

Goal!

So how do those scent molecules get through the air to your nose? They do this by a process called diffusion. The nitrogen, oxygen, carbon dioxide, and other molecules in the air are moving around very fast (about 1000 miles per hour, even faster than a jet airplane!) When a scent molecules gets into the air, the molecules in air begin to hit the scent molecules, kicking it around like a soccer ball. Sooner or later, they kick it into your nose! Now, your nose probably can't smell just one scent molecule, but there are gazillions of air soccer players kicking around billions of scent molecules, so eventually a lot of them get to your nose. Not surprisingly, the farther away your nose is, the longer it takes to smell something. Also, the colder it is, the slower all the molecules move. And the bigger the scent molecule, the longer it takes to smell (usually).

Here's a way you can observe diffusion in action. Line up about 5 or 6 family members or friends in a row, each about 2-3 feet apart. You can do this in a large room, down a hall, or outside. Have them all face away from you and close their eyes. Tell each person to raise a hand when he or she smells something. Open a bottle of something smelly, like mouthwash, perfume, or rubbing alcohol. Turn around and look for raised hands. You will probably see that the people closest to you raise their hands quickly, but that it takes longer for the people farther away. Some may never smell it at all (especially if you're outside on a windy day). Try the same experiment with different kinds of smells and with more people!