The densities of common materials are great enough that molecules cannot travel very far before having a collision that can scatter them in any direction, including straight backward.
It can be shown that the average distance that a molecule travels is proportional to the square root of time:. The quantity is the diffusion constant for the particular molecule in a specific medium. Figure lists representative values of for various substances, in units of.
This decrease is because the average molecular speed at a given temperature is inversely proportional to molecular mass. Thus the more massive molecules diffuse more slowly.
Another interesting point is that for oxygen in air is much greater than for oxygen in water. In water, an oxygen molecule makes many more collisions in its random walk and is slowed considerably. In water, an oxygen molecule moves only about in 1 s. Each molecule actually collides about times per second! Finally, note that diffusion constants increase with temperature, because average molecular speed increases with temperature.
This is because the average kinetic energy of molecules, , is proportional to absolute temperature. Calculate the average time it takes a glucose molecule to move 1. We can use , the expression for the average distance moved in time , and solve it for. All other quantities are known.
Solving for and substituting known values yields. This is a remarkably long time for glucose to move a mere centimeter! For this reason, we stir sugar into water rather than waiting for it to diffuse. Because diffusion is typically very slow, its most important effects occur over small distances. For example, the cornea of the eye gets most of its oxygen by diffusion through the thin tear layer covering it. If you very carefully place a drop of food coloring in a still glass of water, it will slowly diffuse into the colorless surroundings until its concentration is the same everywhere.
This type of diffusion is called free diffusion, because there are no barriers inhibiting it. Let us examine its direction and rate. Molecular motion is random in direction, and so simple chance dictates that more molecules will move out of a region of high concentration than into it. The net rate of diffusion is higher initially than after the process is partially completed. See Figure. The net rate of diffusion is proportional to the concentration difference.
Many more molecules will leave a region of high concentration than will enter it from a region of low concentration. In fact, if the concentrations were the same, there would be no net movement. The net rate of diffusion is also proportional to the diffusion constant , which is determined experimentally. The farther a molecule can diffuse in a given time, the more likely it is to leave the region of high concentration.
Many of the factors that affect the rate are hidden in the diffusion constant. For example, temperature and cohesive and adhesive forces all affect values of.
Diffusion is the dominant mechanism by which the exchange of nutrients and waste products occur between the blood and tissue, and between air and blood in the lungs. In the evolutionary process, as organisms became larger, they needed quicker methods of transportation than net diffusion, because of the larger distances involved in the transport, leading to the development of circulatory systems. Less sophisticated, single-celled organisms still rely totally on diffusion for the removal of waste products and the uptake of nutrients.
Some of the most interesting examples of diffusion occur through barriers that affect the rates of diffusion. For example, when you soak a swollen ankle in Epsom salt, water diffuses through your skin. Comment title. You have entered an invalid code. Submit Cancel. Thank you for submitting a comment on this article. Your comment will be reviewed and published at the journal's discretion. Please check for further notifications by email. View Metrics. Email alerts Article activity alert.
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