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Diffusion is an important concept in elementary science education, especially because it supports (or seems to support) the notion of matter consisting of very small everyday particles (as opposed to a continuous substance).

A colleague of mine is writing an introductory science text for 8th graders. She is struggling to find a good way to introduce the reader to diffusion using an everyday example. We do not like the canonical example of propagation of kitchen smells or perfume, because it is not obvious how to rule out spreading by air convection (i.e. advection of the smelly substance).

So, what's the best example to explain diffusion to an 8th grader?

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A good question, the topic is often fudged. Worst case you may have to use a common example, but say that tho get the real thing you need still air. – dmckee Sep 4 '13 at 14:32
@dmckee yes, that's a standard cheat. – Slaviks Sep 4 '13 at 18:36

Black ink droplet in water should be the most visualizable diffusion example.

The best setup should a big flat disk with radius at least $10cm$. The water being added must be very shallow ($<1cm$) so that the vertical dimension is much small than the other dimension. When you add a droplet in the center, it should appear as cylindrical column so that the system is essentially 2D. In this way, both the gravity effect and convection is suppressed. If the diffusion is isotropic, than it is pretty sure to claim that most effects other than diffusion are suppressed.

For more quantitative results, a light source/laser and light intensity detector (e.g. webcam) can be used to scan through the transparent container. At the beginning, the distribution is like a bump at the center, and as time goes on, it should look like a Gaussian.

To ease the measurement, a very long and narrow container should be used so that the system can be treated as 1D. Then moving the laser-detector back and fro in fixed time intervals can reconstruct the whole distribution $P(x,t)$ in computer. Then they can see whether it is really a Gaussian, and test whether it follows the relation of $\langle x^2\rangle=t$.

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That sounds like very interesting experiment and using a webcam and computer to plot everything and get the students involved is great. – Argus Dec 4 '13 at 10:08

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