Brownian motion of carbon particles in water

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

Apparatus and Materials

Microscope with objective of relatively high power (x40 objective, x10 eyepiece)
Microscope slide with cover slip
Illuminant (lamp, lens and power supply)
Aquadag (colloidal graphite), photographic opaque or Indian ink
Flexicam and screen (optional)

Health & Safety and Technical Notes

Since this will be set up as a demonstration, there is little risk of anyone reflecting the Sun up through the microscope.

Read our standard health & safety guidance

Make a suspension of carbon in water by adding a pinhead-sized speck of colloidal graphite to a few ml of distilled water. The microscope needs a higher power objective than that used in the experiment...

Brownian motion in a smoke cell

...preferably used as a water immersion lens.

Do not leave the water in contact with the objective for any longer than necessary.

Procedure

Observe the Brownian motion of the bits of carbon in the water, by putting one (tiny) drop of solution on the microscope slide and adding the cover slip. If the particles cannot be found, try adding a drop of distilled water to the top of the cover slip and lowering the objective into it.

Teaching Notes

If you are teaching about gases, beware of showing this in place of Brownian motion in air.
Obviously, the particles observed in this experiment will be moving more slowly than fragments of ash in air.
An alternative: use a visualiser with a data projector and screen to enable students to observe Brownian motion in a suspension containing tiny polystyrene spheres.

This experiment was safety-tested in March 2005

This video shows how Brownian motion can be observed in a suspension containing micrometre diameter polystyrene spheres. Using a microscope and video camera, students can observe the motion of the polystyrene spheres. The video also shows how Brownian motion can be simulated using a vibrating loudspeaker, table tennis balls and a small balloon.