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Distinguishes combinations of compounds from true compounds.

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Mr. Brown's Motion

Mr. Brown's Motion


Credit: Mari27454
Source: http://commons.wikimedia.org/wiki/File:Santa_Maria_Maggiore_interno.jpg
License: CC BY-NC 3.0

You’ve probably seen a sunbeam streaming through a window like the windows in this church. What you’re actually seeing is sunlight reflected from tiny dust particles in the air.

The Back Story

  • Did you ever look closely at the dust particles in a beam of sunlight? If you did, you may have noticed that the dust particles were constantly moving in the air. Any tiny particles suspended in a fluid move about randomly, like dust particles in the air.
  • Credit: rpavich
    Source: http://www.flickr.com/photos/rpavich/9473202888/
    License: CC BY-NC 3.0

    You can clearly see the motion of the dust under the light [Figure2]

  • This motion was first discovered by a British scientist named Robert Brown in 1827. That’s why this type of motion is called Brownian motion. Brown observed tiny pollen grains moving randomly in water.
  • What causes Brownian motion? Watch this animation and see if you can figure it out. http://www.aip.org/history/einstein/brownian.htm

Can You Apply It?

Learn more about Brownian motion at the links below. Then answer the questions that follow.

  1. Describe Brownian motion.
  2. Brownian motion occurs in mixtures. What type of mixtures exhibit Brownian motion?
  3. Why does Brownian motion occur?
  4. What size of particles move by Brownian motion? Why do particles of this size exhibit Brownian motion?
  5. What factors affect the speed of Brownian motion? How do the factors affect the speed?
  6. What is the kinetic theory of matter? How does it explain Brownian motion?
  7. Based on what you now know about Brownian motion, predict what would happen if you placed a drop of food coloring in a large pan of water. Assume that you do not stir or disturb the water in any way. Explain your prediction.

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Image Attributions

  1. [1]^ Credit: Mari27454; Source: http://commons.wikimedia.org/wiki/File:Santa_Maria_Maggiore_interno.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: rpavich; Source: http://www.flickr.com/photos/rpavich/9473202888/; License: CC BY-NC 3.0

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