Lesson 1: Kinetic Picture of a Gas

02 Lesson 1: Kinetic Picture of a Gas

The molecules of every substance are in continuous motion, but the kind of motion depends on the state:

If we look through a microscope at the smoke rising from a burning candle, we see tiny carbon particles moving here and there in random zig-zag motion.
This motion is called Brownian motion after the Scottish botanist Robert Brown, who discovered in 1827 that pollen grains suspended in water move constantly and randomly.

Explanation.

The molecules of air (or any gas) move randomly in all directions with different speeds and collide with each other and with the smoke particles.
At any instant the number (or force) of the collisions on one side of a carbon particle may be larger than on the opposite side, so the particle is pushed a short distance in one direction.
A moment later the unbalance is in another direction.
The motion of the molecules is caused by heat and is never-ending.

Conclusion.

Gas molecules are in continuous random motion; they collide with each other and with the walls of the container.
(The collisions with the walls are the cause of gas pressure.)

Experiment.

Hold a beaker full of ammonia gas (NH₃) upside down over a beaker of hydrogen chloride gas (HCl) (with a cover separating them, which is then removed).
A white cloud of ammonium chloride (NH₄Cl) forms and spreads until it fills both beakers.

Explanation.

HCl molecules, although heavier (denser), rise upward through the spaces between ammonia molecules; NH₃ molecules, although lighter, move downward through the spaces between HCl molecules.
Where they meet, they combine to form NH₄Cl, which diffuses to fill both beakers.

Conclusion.

There are relatively large spaces between gas molecules (intermolecular spaces).
This also explains why gases are easily compressed: pressure pushes the molecules closer together, so the volume decreases.