The Restless Dance

Why it matters

Look at a beam of sunlight in a dusty room. The tiny specks never settle; they keep jittering in a restless zig-zag dance, pushed about by air molecules you cannot see.

Predict first

Under a microscope, tiny pollen grains floating on still water keep jiggling in random zig-zags. What is pushing them?

The restless zig-zag movement of tiny particles is called Brownian motion. It happens because invisible, fast-moving molecules of water or air keep striking the specks from every side.

• A hidden breeze blowing across the water
• Fast-moving water molecules constantly bumping into them from all sides — correct
• The grains are alive and swimming
• Nothing; they only look like they move

What you do

Sort each situation by whether it shows that particles are always moving, or does not show it.

Check yourself

What is Brownian motion?

Brownian motion is the jerky, random, zig-zag movement of very small particles like smoke or pollen. It happens because invisible fast-moving molecules keep bumping into them.

Brownian motion is important because it is evidence that...

Because the specks are pushed about by unseen molecules, Brownian motion is direct evidence that the molecules of gases and liquids are always moving, even when the liquid looks still.

If you gently WARM the water, what happens to the Brownian dance of the pollen grains?

Warming the water gives its molecules more energy, so they move faster and strike the pollen grains harder and more often. The Brownian motion becomes quicker and more lively.

• The random zig-zag movement of tiny particles caused by being hit by moving molecules — correct
• The steady sinking of heavy particles to the bottom
• Particles lining up neatly in a row
• Particles freezing completely still
• The molecules of liquids and gases are in constant, restless motion — correct
• Particles never move at all
• Only large objects can move
• Heat has no effect on particles
• It becomes faster and more vigorous, because the molecules move faster — correct
• It stops completely, because heat calms particles
• It stays exactly the same, because heat does nothing
• The grains line up in a straight row