Brownian Motion

Brownian motion is the random, zig-zag movement of tiny visible particles (like pollen or smoke) when they are suspended in a liquid or a gas. This motion is clear evidence that matter is made of constantly moving particles (atoms or molecules).

What you observe

• Under a microscope, small specks in water or air jerk about in unpredictable directions.
• The motion never follows a smooth path and changes from moment to moment.

Why it happens (kinetic particle model)

Atoms or molecules in liquids and gases move quickly and randomly. They constantly collide with any tiny particle suspended in them. These collisions are uneven, so the suspended particle gets nudged from different sides at different times, producing a zig-zag path.

Analogy: Imagine a beach ball on a crowded pool being knocked by many smaller balls from all directions. The beach ball moves randomly because the hits are unequal.

Use terms correctly

• Atoms or molecules are the tiny building blocks of matter (too small to see).
• Microscopic particles are the small specks you can just see under a microscope (like pollen, dust, or smoke). These are moved by collisions with atoms or molecules.

Where and how to observe

• Pollen grains in water under a microscope.
• Smoke particles in air in a smoke cell (seen as jittery dots).

Light lets you see the particles; it does not make them move. The movement is caused by collisions with invisible atoms or molecules.

What affects Brownian motion

• Higher temperature: atoms/molecules move faster, so the motion becomes more vigorous.
• Smaller suspended particles: easier to jolt, so movement is more noticeable.
• Thicker (more viscous) fluids: movement is reduced.
• No fluid (a vacuum): no collisions, so no Brownian motion.

Common misconceptions

• “The microscope lamp makes them move.” False: motion is from collisions with atoms/molecules.
• “Currents cause the motion.” In a still fluid, particles still jiggle randomly.
• “It should settle and stop.” Collisions continue, so motion continues while the fluid’s particles keep moving.

Key points

• Brownian motion is random motion of microscopic particles in fluids.
• It is caused by uneven collisions with fast-moving atoms or molecules.
• It provides evidence for the kinetic particle model of matter.