12.7: Henry's Law
- Describe the effect of pressure on the solubility of gases in water and apply this concept in health-related conditions.
Solubility of a Gas in Water
Earlier you were introduced to the effect of intermolecular attractive forces on solution formation. The chemical structures of the solute and solvent dictate the types of forces possible and, consequently, are important factors in determining solubility.
For example, under similar conditions, the water solubility of oxygen is approximately three times greater than that of helium.
The solubility of oxygen (nonpolar) in the liquid hydrocarbon hexane, C 6 H 14 (nonpolar), is approximately 20 times greater than it is in water (polar).
Solubility of a Gas and Temperature
Temperature is another factor that affects gas solubility. Gas solubility decreases as temperature increases. This is one of the major impacts resulting from the thermal pollution of natural bodies of water. When the temperature of a river, lake, or stream is raised abnormally high, usually due to the discharge of hot water from some industrial process, the solubility of oxygen in the water is decreased. Decreased levels of dissolved oxygen may have serious consequences for the health of the water’s ecosystems and, in severe cases, can result in large-scale fish kills.
Henry's Law (Solubility of a Gas and Pressure)
The solubility of a gaseous solute is also affected by pressure. Gas solubility increases as the pressure of the gas increases. Carbonated beverages provide a nice illustration of this relationship. The carbonation process involves exposing the beverage to a relatively high pressure of carbon dioxide gas and then sealing the beverage container, thus saturating the beverage with CO 2 at high pressure. When the beverage container is opened, a familiar hiss is heard as the carbon dioxide gas pressure is released, and some of the dissolved carbon dioxide is typically seen leaving solution in the form of small bubbles (Figure \(\PageIndex{1}\)). With time, the dissolved carbon dioxide concentration will decrease to its equilibrium value and the beverage will become “flat.”
Decompression Sickness (“The Bends”)
Decompression sickness (DCS), or “the bends,” is an effect of the increased pressure of the air inhaled by scuba divers when swimming underwater at considerable depths. In addition to the pressure exerted by the atmosphere, divers are subjected to additional pressure due to the water above them, experiencing approximately 2 atm at 10 m of depth. Therefore, the air inhaled by a diver while submerged contains gases at the corresponding higher pressure, and the concentrations of the gases dissolved in the diver’s blood are proportionally higher per Henry’s law.
As the diver ascends to the surface of the water, the atmospheric pressure decreases and the dissolved gases becomes less soluble. If the ascent is too rapid, the gases escaping from the diver’s blood may form bubbles that eventually bursts capillaries and cause a variety of symptoms ranging from rashes and joint pain to paralysis and death. To avoid DCS, divers must ascend from depths at relatively slow speeds (10 or 20 m/min) or otherwise make several decompression stops, pausing for several minutes at given depths during the ascent. This allows excess dissolved gases in the blood to be exhaled. If stops or slow speeds are not an option, then the diver should exhale rapidly during the ascent.
Divers may also use a special mixture of helium and oxygen. Nitrogen in air is replaced by helium. He gas is not as soluble in the blood serum, so the effects of DCS is minimized. Under similar conditions, the water solubility of nitrogen is greater than that of helium.
When these preventive measures are unsuccessful, divers with DCS are often provided hyperbaric oxygen therapy in pressurized vessels called decompression (or recompression) chambers.
Hyperbaric Chamber
Hyperbaric (high pressure) oxygen treatment in another application of Henry's Law. This is an air chamber that is at two to three times the atmospheric pressure. The solubility of oxygen increases with an increase in pressure. A patient placed in a hyperbaric chamber has a higher concentration of oxygen dissolved in blood. The higher concentration of oxygen is toxic to many strains of bacteria. Therefore the hyperbaric chambers are used to treat burn patients, gangrene, tetenus, in surgeries, and to treat some cancers.
The hyperbaric chambers are also used to treat carbon monoxide (CO) poisoning because the higher concentration of oxygen in the chamber can displace the CO bound with hemoglobin faster than atmospheric oxygen does.
Another use of hyperbaric chambers is to treat scuba divers suffering from the bends. If a diver ascends too quickly, the nitrogen dissolved in blood makes bubbles in the vessels that block the blood flow, a condition called bends. The divers suffering from the bends are placed in a hyperbaric chamber at high pressure, and then the pressure is slowly decreased to atmospheric pressure. The nitrogen dissolved in blood under higher pressure and slowly diffuses out through the lungs as the pressure is gradually decreased.
Figure \(\PageIndex{2}\): (a) US Navy divers undergo training in a recompression chamber. (b) Divers receive hyperbaric oxygen therapy.
Summary
The solubility of a gas in a liquid is proportional to the pressure of the gas over the liquid. The higher the pressure of a gas above a liquid, the greater its solubility.
Contributions & Attributions
- Wikipedia
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Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110 ).
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Henry Agnew (UC Davis)