BROWNIAN MOVEMENT ACTIVITY

Heat causes random, passive motion of molecules. Even though the movement of molecules cannot be seen, the movement of small particles (approximately 1-100 nanometers) can be seen after their collisions with the moving molecules. The motion of the suspended particles is called Brownian movement (or motion) and is visible under high power
magnification.

India ink is a good source of small particles for you to investigate. You will see that the ink is not a solution, but actually a suspension of black pigment particles. These particles are small enough to vibrate when water molecules bump into them.

PROCEDURE:

1. Place a drop of dilute India ink onto a microscope slide and cover with a cover slip.

2. Focus at low power.

3. Switch to high power, being careful to not get ink on the objective lens.

4. As you focus, the drop will go from a uniform gray color to clear individually-vibrating ink particles.

5. Leave the slide lit for 2 minutes and record any change that you see in the particles behavior.

QUESTIONS:

1. Describe your observation of the ink particles.
2. Why are the ink particles visible, but the water molecules are not?
3. How does heat affect particle movement?
4. List reasons that you think might cause the particles to not move.

(TEACHER’S NOTE: This is adapted from Voldopich and Moore, Biology Laboratory Manual, 1992. Additional diffusion and osmosis exercises are in their manual. A diffusion, osmosis and membrane lab is available on-line at http://www.rit.edu/~gtfsbi/IntroCB/lab3.htm.

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