one another. Eventually each component gas will uniformly occupy the space that confines them. Because of diffusion, the nitrogen, oxygen, argon, etc. in the air that you breathe or the air in a scuba cylinder are each equally distributed. When additional oxygen is added to air to make enriched air nitrox, the oxygen distributes itself throughout the nitrox mix. It does not puddle in the bottom of the cylinder. If a gas is introduced on one side of a permeable membrane (that is, a barrier that permits the gas molecules to diffuse through it) or if the pressure of the gas is increased on one side of the membrane, then molecules will pass through the membrane from the area of higher pressure to the area of lower pressure. When the pressure on both sides of the membrane becomes equal, there will be no further change; a state of equilibrium will have been reached. Because of their random motion, molecules are continually passing through the membrane in both directions. But as long as there is more pressure on one side of the membrane than on the other, more molecules will move from the area of higher concentration to the area of lower concentration. The greater the pressure gradient, or difference in pressure, the more rapid is the move toward equilibrium. As the pressure gradient drops, so does the rate at which the system approaches equilibrium. A similar thing occurs when a gas is in contact with a liquid. Gases will dissolve in liquids just as solids will. You know that sugar dissolves in water because you can see it happen. Most persons are less aware that gases also dissolve, although carbonated beverages are a perfect everyday example of gas solubility. Some gases are more soluble in a liquid than other gases, and some liquids are better solvents of a gas than other liquids. This also parallels your own experience with different solids dissolving in different liquids. Nitrogen, as an example relevant to diving, is five times more soluble in fat than in water. The amount of a gas that will dissolve in a liquid is dependent on the pressure of the gas on the liquid, as described in Henry’s law: “At a given temperature, the weight of a gas dissolved by a liquid is directly proportional to the partial pressure of the gas upon the liquid Chapter 3- Diving Physics (figure 3-9).” The higher the partial pressure of a gas on a liquid, the more gas will dissolve in the liquid. Henry’s law also is an “ideal gas” law, but it holds for dilute solutions and low gas pressures. According to Henry’s law, the relationship is linear. If one quantity of gas will dissolve at one atmosphere of pressure, then three quantities of gas will dissolve at three atmospheres. Temperature also affects the quantity of a gas that will be absorbed by a liquid. The solubility of a gas is inversely related to the temperature–the higher the temperature, the lower the solubility and vice versa. As the temperature of a solution is increased, the dissolved gas molecules become more active and begin to escape from solution. You witness this when you heat water. Bubbles begin to form in the water and escape long before the water begins to boil. A carbonated beverage in an open container in the refrigerator takes longer to become “flat” than one at room temperature. Henry’s law addresses the quantity of a gas that will dissolve, but it does not describe the rate at which the gas will dissolve. This diffusion is similar to the permeable membrane model above. Whenever a gas is brought into contact with a liquid or if the pressure of the gas on the liquid is increased, molecules of gas begin to diffuse into the liquid, and we say ingassing occurs. In the beginning, the gas moves rapidly into solution, driven by the relatively high partial pressure of the gas on the liquid. There are simply many more gas molecules available to Diving Physics 95 Solubility of Gases 1. Equilibrium = 1 ATM 2. Non-equilibrium with pressure increased 3. Equilibrium at Increased Pressure 4. Non-equilibrium with pressure decreased The amount of a gas which dissolves in a liquid is proportional to the pressure of the gas in contact with the liquid FIGURE 3-9. HENRY’S LAW
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