Air is mostly empty space, which should not surprise you. But you have felt the wind (which is air movement) on your face, so you know that something is there. That something is a mixture of molecules, including nitrogen (about 78%), oxygen (~ 21%), argon (~1%), and traces of carbon dioxide and pollution (less than 1%).
Molecules of air take up less than one two-thousandth of the air's space. For example, one cubic centimeter of dry ice (solid carbon dioxide, the size of a sugar cube) evaporates (sublimes, actually) to fill a two-later bottle with carbon dioxide gas (pushing out all the air if the bottle is open, bursting the bottle if it's not). So the two-liter bottle of gas contains only a sugar-cube's worth of solid.
But air, though almost empty, is a busy place. The molecules are traveling around at an eye-catching (if they were visible) speed of about one thousand miles per hour, crashing into each other and everything else. I'm serious. Good thing they are small. If we could see them, and exaggerate the size of the molecules, it might look something like this:
Because this does not show their movement, the molecules are colored to help you imagine the motion. Think of the red ones as moving the fastest, the blue ones the slowest, and the other colors somewhere in between. They are always colliding and changing speeds (exchanging energy), but the average speed is about 1000 mph.
You can easily feel the push of all these collisions. Blow up a balloon. Tie the mouth closed. Squeeze the balloon. You are pushing back at countless* collisions of molecules with the inside of the balloon. And the molecules are so small, that it does not even matter what they are. A sample of oxygen behaves exactly the same way as the same size sample of nitrogen, or even a mixture of the two. They all behave like what chemists call an ideal gas.
The force you push against is called air pressure. By blowing up the balloon, you forced more air into it than there are now outside it. So there are more collisions against the inside of the ballon than against the outside. Those extra collisions support the ballon -- they keep it stretched. Air pressure is the sum of countless collisions of air molecules with whatever is around. Air pressure pushes against every surface with a force of about 15 pounds on every square inch (think postage stamp) of that surface. That would crush you, except that air pressure inside you is pushing back with just the same pressure. Whew! Dodged that crushing blow.
By the way, that fifteen pounds is the weight of the postage-stamp sized column of the air up to the top of Earth's atmosphere at about 6000 miles up. Air pressure is just gravity at work, keeping those 1000-mph molecules down to earth.
Finally, squeezing the balloon tells you that air is springy. Take the squeeze away, and the balloon springs right back. Hold the balloon tightly by its side in one hand. Turn it on its side, and lay a small piece of paper (think half a stamp) on top. Firmly thump the balloon from the bottom, trying to keep the balloon itself from moving. It you do it just right, the paper will leap off the surface. That suggests that a vibration generated by your thump has travelled through the air in the balloon (or perhaps around the balloon through the rubber), sort of like a wave travels through water.
But wait. A wave traveling through air? That's what sound is. Unit 2 is about sound.
Resources (Take a little or a lot, there's more than just about anyone wants to know.)
• Air, according to Wikipedia
• Air Pressure, according to Wikipedia
• Simulation of air particles in motion, as used in class
This simulation requires the latest version of Java, which causes problems for just about everyone.
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* Well, not really countless, but pretty hard to count. That two liters of carbon dioxide gas contains very roughly 36,000,000,000,000,000,000,000 molecules. For those of you who speak a little bit of mathematics, that's 3.6 x 1022 molecules. Might as well say countless.
