Brownian motion of carbon particles in water
Practical Activity for 14-16
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
Flexicamand 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.
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...
...preferably used as a water immersion lens.
Do not leave the water in contact with the objective for any longer than necessary.
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.
- 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.
This video shows footage of the movement of particles by Brownian motion and can be used in the classroom with your students: