4.Main Content

experiments

Brownian motion of carbon particles in water

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)

Technical notes

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.

Safety

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

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.
 
This experiment was safety-checked in March 2005