"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!
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