What's important about air and gases?

The molecules are not in the gas, they are the gas. There isn't anything else. The vast majority of the volume of a gas actually is literally nothing.

By far the most important thing about gases is simply that they exist. They are not nothing, but are another form of "stuff" (matter) just like solids and liquids. They have mass and are comprised of molecules.

The main reason gases seem so insubstantial is that they're typically much less dense — about a thousand times less — than solids and liquids. That low density means that the molecules in a gas are much farther apart than in solids and liquids — about ten times farther apart on the average — which means they're not attached to each other, and in fact are pretty much non-interacting, except when they bounce off each other. The lack of interaction, in turn, makes gases actually much easier to understand at a fundamental level than solids or liquids. That's why there's an "ideal gas law" but no "ideal solid law" or "ideal liquid law".

If the molecules are that far apart, then what's in between them? Nothing. Really. The molecules are not in the gas, they are the gas. There isn't anything else. The vast majority of the volume of a gas actually is literally nothing. All the mass is in the molecules, but at any given moment they take up only about one thousandth of the volume. But keep in mind that "far apart" here is only relative to solids and liquids. The average distance between molecules in air is about one ten-millionth of an inch — it's not like you can walk between the molecules. Also the molecules are in constant, rapid motion — a typical molecular speed in air is several hundred miles per hour. So even though at any given instant the molecules occupy a small fraction of the volume, the "empty" spaces have molecules passing through them all the time.

Under the right conditions — low enough temperature and/or high enough pressure — any gas can condense into a liquid. We see this with water vapor when it condenses on a cold surface. But even air itself can be liquified; it just requires very low temperatures, far below anything that occurs even in the coldest places on earth. Still, with the right equipment it's not that hard to do. Liquid nitrogen is cheaper than milk.

Finally, air is not the only gas. The bubbles in boiling water, for example, contain essentially pure water vapor. Air itself is a mixture of gases (about 4/5 nitrogen and 1/5 oxygen, with much smaller amounts of argon, water vapor, carbon dioxide and other stuff), though this isn't something we're emphasizing in the present curriculum.

—Roger Tobin