When you burn something on the stove, the kitchen will smell smoky. A few minutes later, though, your whole place will smell of the burnt food. That's because the atoms of burnt food diffuse through your home. Diffusion is the process by which atoms of one material are transferred into another material through random atomic motion. In diffusion, atoms tend to spread themselves evenly, as when the smoke moves from the high concentration in the kitchen to a lower concentration all through your home. The diffusion rate depends on several factors.
Of all the factors that influence diffusion rate, temperature is the most important. Temperature has the greatest effect on diffusion rates and is the easiest of the factors to change. Increasing the temperature increases the diffusion rate by adding energy to each particle. This is because particles with more energy bounce against each other more frequently and spread evenly throughout the material volume. Similarly, lowering the temperature will lower the diffusion rate by lowering the energy of each particle.
The rate of diffusion depends on the difference between concentrations across the host material, with higher concentration differences resulting in higher diffusion rates. For example, diffusion through a thin wall or membrane will occur quickly if there is a high concentration of the gas on one side and none of the gas on the other side of the wall. If there is already an almost equal amount of gas on both sides, diffusion will be much slower.
The rate of diffusion is inversely related to the distance through which the material is diffusing. That is, smaller distances result in faster diffusion rates and larger distances result in slower diffusion rates. This makes sense, since a gas diffuses through a thin wall much faster than it would diffuse through a thick wall.
Diffusing and Host Materials
Diffusion rate also depends on both the material that is diffusing and the material it is diffusing through. At a certain temperature, all particles have the same average energy. This means that lighter atoms, such as hydrogen, carbon, oxygen and nitrogen travel faster and are more mobile than larger atoms such as copper or iron. Materials made of these lighter atoms diffuse faster than heavier materials.
- "Materials Science and Engineering: An Introduction 8th ed."; William Callister Jr.; 2010
- University of Tennesse, Knoxville: Diffusion
- Photo Credit Irene Treston-waller/Hemera/Getty Images
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