MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Methylene blue dye diffusing through agar gel in a test tube. The test tube at left is at the start of the experiment, at right is after one week. A sample of the dye was placed in a thin layer half way down the tube. The dye has started to dissolve into the agar and permeate throughout the tube. Diffusion occurs because all molecules have motion due to their heat energy. Each molecule moves in a random direction, meaning that there will be a net movement of dye molecules from areas of high concentration (the central band) into areas of low concentration (the agar). The rate of diffusion depends upon the nature of the substances involved, and their temperature.
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