That Sinking Feeling

Helium-filled balloons are lots of fun because the balloon lifts up into the air. But they don't last long. Within a day or two, they begin to sink lower and lower, and to get smaller and smaller. Why does this happen?

This is an example of a process known as effusion. Effusion occurs when gas molecules are able to escape their container through a little hole. All gas molecules, whether they're heliums in the balloon, or nitrogens, oxygens, or something else in the air, are moving around really fast (around 1000 miles per hour!!). Every now and then, one of them finds a hole and gets out. Understand that these are very, very tiny holes - too small for you to see. Helium atoms are much smaller than molecules like nitrogen, oxygen, or carbon dioxide, so it's a lot easier for them to get out the hole. They also move faster, so they find the holes more quickly.

You might try some experiments with effusion. Get two helium-filled balloons - one rubber and one Mylar (the shiny kind). Do they both lose helium at the same rate? Compare a helium-filled balloon to one inflated with air or your breath (which has more carbon dioxide than the air you breathed in). Can you see the difference in gas effusion?

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!