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| Evidence for Particulate Nature of Matter From Brownian Motion |
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| When we look at a glass of still water on the table, it seems incredible that millions of molecules are moving about restlessly inside it. A British botanist called Robert Brown, however, discovered evidence of this movement over 200 years ago. He was observing through a microscope some very tiny particles of pollen in suspension in water. To his surprise he saw that some of the smaller particles were moving about continuously in a haphazard way. It is due to the bombardment of tiny particles by the liquid molecules all around it. The direction of the resultant force is constantly changing. Hence the motion of the particles is completely random. Thus 'Brownian motion' is defined as the random or zig zag motion of the suspended molecules. |
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| Brownian motion can be demonstrated simply by releasing some smoke particles from burning cord into a small glass container and putting a cover plate to seal the smoke and air into the cell. |
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| To investigate liquid molecular movement place some water with graphite particles suspended in it in the cell. |
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| Now adjust the microscope slightly until you can see very bright specks. The particles of graphite (or smoke) scatter (reflect) the light shining on them and so appear as bright points of light darting about in a random or erratic motion. Note that the graphite (or smoke) particles are much larger than the water (or air) molecules. The particles can be seen by the light they scatter but the molecules themselves are too small to be seen. |
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| The irregular movement of the visible particles of graphite (or smoke) is explained as being due to an uneven bombardment of the particles by the invisible molecules of water (or air). It is due to Brownian motion. |
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| Brownian motion |
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| We can conclude that lighter the particles faster the motion and denser the particles slower the motion. |
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