Brownian Motion in a Smoke Cell

This classic experiment demonstrates Brownian motion by observing tiny smoke particles under a microscope. Their random, jittery movement provides strong evidence for the particulate nature of matter and the constant motion of gas molecules.

1. Generate smoke using a paper straw or similar safe source.
2. Fill a smoke cell with the smoke and cover it with a glass cover-slip to reduce loss of particles.
3. Place the smoke cell on the microscope stage and connect the light source to a low-voltage power supply.
4. Focus the microscope carefully until tiny bright specks (smoke particles) come into view.
5. Observe the movement of the specks over time, noting their random jiggling and occasional disappearance as they move in and out of focus.

A Smoke Cell demonstrating Brownian Motion in Air. - FranklyChemistry:

Observing Brownian Motion with a Smoke Cell - Physics with Simon Poliakoff:

📄 Brownian motion in a smoke cell - Institute of Physics: https://spark.iop.org/brownian-motion-smoke-cell

• Use a camera or projector so the entire class can observe at once.
• Replace smoke with polystyrene microspheres suspended in water.
• Demonstrate the idea using a loudspeaker vibrating table tennis balls and a balloon as a large-scale analogy.

• Do not allow sunlight to reflect up through the microscope.
• Use only small, controlled sources of smoke such as paper straws; avoid hazardous or plastic materials that may release harmful fumes.
• Clean the smoke cell regularly to maintain visibility and reduce residue build-up.
• Handle glassware with care to avoid breakage.

• Why do the smoke particles appear to jiggle randomly? (They are being bombarded by invisible air molecules moving at high speeds.)
• Why might some particles drift slowly in one direction? (Large-scale convection currents or uneven air movement, which are not part of the true Brownian motion.)
• How does this experiment support the kinetic theory of gases? (It provides evidence that air molecules are real and in constant motion, even though they cannot be seen directly.)
• How small must air molecules be compared to smoke particles? (Much smaller—orders of magnitude smaller—since they can move visible smoke particles through collisions.)