Brownian motion: evidence that molecules exist

0:00Molecules are too small to see, even with a light microscope. So how do we know they exist?

0:07Under a light microscope, we can see tiny grains floating in water.

0:13Each grain jiggles and wanders at random, and it never stops.

0:18This is called Brownian motion, after Robert Brown, who saw it in 1827.

0:24Brown first saw it in grains from pollen, so he wondered if they were alive.

0:30But grains of dust and rock jiggle the same way, and they are not alive.

0:36So what pushes them? Let's zoom in on one grain. This drawing is not to scale.

0:43Water is made of molecules, tiny groups of joined atoms that are always moving.

0:50They hit the grain from all sides, all the time.

0:54At any moment, more molecules hit one side than the other, so the grain gets an overall push.

1:02Which side gets more hits keeps changing, so the grain jiggles at random.

1:09In 1905, Albert Einstein explained Brownian motion this way and predicted how far grains should wander.

1:17In 1908, Jean Perrin's experiments matched those predictions.

1:22This convinced most scientists that atoms and molecules really exist.

1:28It also supports kinetic molecular theory: matter is made of tiny particles that are always moving.

1:35So grains jiggle because moving molecules hit them unevenly, which is strong evidence that molecules exist.