Diffusion commonly refers to the spontaneous movement of a substance (gas, liquid, or solid) into its surrounding area. The molecules, or particles, that make up the substance distribute over time from an area of higher concentration to an area of lower concentration in order to create, at equilibrium, a uniform distribution of particles throughout the system. Diffusion is a natural process that requires no added energy to occur. It increases the entropy of the system and hence is an energetically favorable and irreversible process.
An example of diffusion is the release of a drop of ink into a beaker of water. The ink will be visibly distinguishable from the water for some amount of time, but it will diffuse eventually to all areas of the beaker. The collision of the ink molecules with the water molecules keeps diffusion from happening quickly. This is an example of a liquid diffusing into another liquid. In comparison to a liquid, a gas has a much lower density. With fewer molecules with which to collide, diffusion of gases occurs much more quickly than diffusion of liquids (or solids). Comparing two gases, the lighter gas (i.e., the gas with the smaller molar mass) diffuses faster that the heavier gas (i.e., the gas with the larger molar mass).Solids diffuse into one another as well, but at an even slower rate that liquids. The molecules in a solid cannot move very much at all. Not only does a solid have greater density compared to a liquid or gas, resulting in the very short movement of one molecule before its collision with another, but energetically favorable intermolecular interactions of a solid also slow diffusion. As a result, diffusion of a solid into another solid takes place over a very long period of time.
Temperature also influences the rate of diffusion. Temperature is a measure of the average thermal energy of the system. As the thermal energy increases, the molecules move faster. Therefore, diffusion occurs faster at higher temperatures.
Scottish chemist Thomas Graham (1805-1869) measured the relative ability of substances to pass through a membrane. Graham's law of diffusion states that the rate at which a gas diffuses is directly proportional to temperature, but inversely proportional to the square root of the molar mass of the gas. In other words, the smaller the molar mass of the gas, the faster it diffuses.
Common examples of diffusion include the diffusion of solids into liquids, such as salt into water. The formation of a solution of the salt in the water by this process is called dissolution. A liquid may diffuse into a gas (such as water into air) by the process of evaporation, or a solid into a gas (such as camphor into air) by the process of sublimation.Osmosis is the diffusion of a liquid (the solvent) across a semipermeable membrane. The term semipermeable refers to the ability of the membrane to prevent passage of the solids that are dissolved in the liquid (the solutes) through the membrane. Osmosis is important in the extraction by plants of water from the soil.
Dialysis, also discovered by Graham, is an important application of diffusion. In dialysis a solution is passed over a semipermeable membrane, allowing solutes up to a certain size (but not larger molecules) to diffuse across the membrane to a second solution. Artificial kidney machines use dialysis to remove metabolic waste products, such as urea and creatinine, from the blood. In these machines, blood is circulated on one side of a semipermeable membrane (made from cellophane), while a dialysis fluid, which closely matches the chemical composition of blood, is circulated on the other side of the membrane. The waste products diffuse from the blood into the dialysis fluid and are then discarded. Important blood components, such as the oxygen- carrying protein hemoglobin, are too large to enter the pores of the membrane and hence are retained in the blood.
A final example of the biological importance of diffusion is the exchange of gases to and from the blood that occurs in the alveolar membrane of the lungs. This membrane separates the flowing blood from the gases within the lung.Carbon dioxide (CO2), a chemical end product of biological metabolism, is plentiful in venous blood that enters the lung. Release of the CO2 from this blood occurs by its diffusion across the alveolar membrane, and this CO2 is expelled upon exhalation. Inhalation brings air into the lung, and air contains 20.95% by volume of oxygen (O2). Diffusion of O2 across the alveolar membrane, in the other direction, allows its dissolution in the blood. Oxygen is carried, bound to hemoglobin, by the arterial blood to the cells where it is released--again by diffusion--for its use by the cells as the terminal oxidant of aerobic respiration.
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