Air molecules collide billions of times per second yet occupy little volume

Evidence

Aristotle could not have known that air is a seething mass of molecules crashing into one another. More than a billion collisions are suffered by every molecule every second at sea level and room temperature. In spite of all the collisions, air is mostly empty space: the molecules occupy only about one part in 5000 of the available volume. Think of 10 bumper cars in an area the size of a football field. You might think that this was a relatively safe, low density of cars—unless each car was traveling at thousands of kilometers per hour. There would be many collisions every second. Between collisions, molecules speed along a straight path at typically half a kilometer per second, managing to travel only a tenth of the length of a typical bacterium before suffering another collision. — Eric J. Heller, Why You Hear What You Hear- An…, 48

Categorization

• CANDY: The bumper-cars-on-a-football-field analogy plus the bacterium-scale free path is a memorable cocktail-party fact.
• XREF: Connects to kinetic theory of gases and the molecular model of sound propagation in any physics-of-sound treatment.