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Particle theory Diffusion Brownian motion

As we have found in the last chapter, particles are always moving. However, how exactly would they move? A common phenomenon observed from the movement of particles is diffusion. Diffusion means a kind of particles going from a place of higher concentration to a place of lower concentration. Imagine a herd of lions and a herd of tigers mixed together. If diffusion happens, lions will go to a place with more tigers than lions, and tigers will go to a place with less tigers than lions. At the end, the tigers and lions will be mixed more thoroughly. That is what particles are like during diffusion.

Examples of diffusion

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What diffusion looks like.

If you dip a paintbrush with a bit of red paint on the tip into a cup of water, the paint will spread all over the cup. The red will spread to places that are more transparent, while the transparency spreads to places that are redder. At the end, the paint and water will be thoroughly mixed so the water is red. (Here we assume that the paint is soluble in water.)

Another example is smell. Imagine a piece of stilton cheese is placed on a table in a corner of a room. You are standing on the opposite corner, a friend of yours is standing in the centre, and another friend is standing right next to the stilton cheese. You must be the last to smell it.

Perfume in a plastic bag

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This experiment is inspired from an experiment from the following source:

  • Chan, W. K.; Luk, W. Y.; Kong, S. W. "Matter as particles". Understanding Integrated Science. Vol. 1B. Aristo Educational Press LTD. p. 131–132.
  1. Put a drop of perfume into a plastic bag, then tie up its opening.
  2. Sniff. You can't smell perfume yet.
  3. Wait a little bit, sniff again. You should be able to smell perfume now.

This experiment proves that there are spaces between the particles of the wall of the plastic bag, and that the perfume diffuses. When the perfume on the bottom of the bag diffuses, it flies to the wall of the bag. As the perfume particles are small enough, they escape through the gaps between the plastic particles and into your nose.

If A and B are to meet, do you think A will come to B or B will come to A? Why?

Answer: B will come to A. This is because A is in a place of lower concentration while B is in a place of higher concentration. B will flow to A's place because of diffusion.