Perhaps you have had the unpleasant experience of walking down the street and being accosted by the noxious smell of rotten eggs emanating from a nearby sewer. The odor, which is produced by the gaseous compound hydrogen sulfide, will reach your nose even if there is no wind.
This experience confirms the existence of gases in air, and the theory that the particles in a gas are constantly in motion. If they were not, odors would not carry unless there was a wind current present. As the Roman poet Lucretius said 2000 years ago in his epic poem The Nature of Things:
We can perceive the various scents of things
Yet never see them coming to our nostrils
The scientific model for gases is that of independent, tiny particles traveling rapidly in straight-line motion through empty space, as a rocket ship travels through outer space. Owing to the rapid motion of its constituent particles, a gas quickly expands to fill completely whatever space is accessible to it.
As a given gas particle travels through space, it occasionally collides with other gas particles or with the walls of its container if it is in one. These collisions result in a change in direction for the particles—much as a billiard ball changes direction when it hits another ball or hits the side of the pool table.
One of the many pieces of evidence that led to the scientific model for gases is that gases are much easier to compress to a smaller volume than are liquids or solids. Compressing a gas corresponds only to reducing the amount of empty space that lies between the independent particles.
A piece of evidence that led to the notion that the particles in a gas are in constant motion is the fact that a gas exerts a force on the walls of whatever container it occupies. Technically, the pressure exerted by a gas is the amount of force that it exerts on a specified area of surface, say one square centimeter (see Figure a).
For example, the helium gas atoms in a helium-filled balloon are in constant motion. As a consequence of their movements, they often collide with the inside skin of the balloon (see Figure b).
The pressure exerted on the balloon walls by this constant bombardment is sufficient to keep the balloon “blown up,” even though the stretched elastic of the balloon’s material is trying to contract and thereby collapse the interior. Indeed, the balloon collapses only when some of the helium leaks into the air outside the balloon.
