Unit 6


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6.1 - The Diffusion Chamber

6.2 - Periodic Precipitation: Liesegang Rings

How do chemical and biological reactions take place? Atoms and molecules have no legs. They cannot walk around and come together to form new molecules and the fractal patterns that we have been studying. How do they find other particles with which to react?

One answer to this question is mixing. Air circulates in a room due to differences in temperature and the stirring as people come and go. But chemical and biological reactions go on even in stillness and in cell structures where physical mixing is small or absent. In such circumstances, atoms and molecules get their legs from diffusion. Diffusion is the spread of atoms and molecules due to their random motion from regions of greater concentration into regions of smaller concentration. Without diffusion, life as we know it would be impossible. This unit describes the process of diffusion.

Imagine the following scenario (as shown in Figure ): In one corner of a school laboratory, Barry opens a bottle of ammonia. Jennifer, who is sitting in the opposite corner of the classroom, grabs her nose and shouts, "Barry, close that bottle!'' Barry does so, but Jennifer continues to smell the ammonia, so she sensibly walks out of the laboratory.


Figure 6.1: When does Jennifer smell the ammonia?

Q6.1: Discuss this Ammonia Story. What questions do you have about it? Discuss answers to your questions and answer the questions below:

A quick guess: How soon will Jennifer smell the ammonia in still air?

What is carrying the odor to Jennifer? What moves across the room?

Whatever carries the odor across the room: How does it travel? in a straight line? along a jagged path? Or does it simply ride with the air as the air circulates?


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