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10.2: Solubility and Pollution

  • Page ID
    209045
  • A solution may be gases, liquids, or solids and is composed of a solute and solvent. The solvent is usually the part of the solution that is present in the greatest amount. The solute is the substance that is dissolving in the solvent and is present in a smaller amount. Solubility is the amount of solute needed to form a saturated solution for a given quantity of solvent at a given temperature to produce a saturated solution.1 Solubility versus temperature dependent graphs are often created and show that for most ionic substances their solubility increases with temperature. In addition, the solubility of ionic compounds at a particular temperature varies due to interactions between the solute and solvent. In addition, solubility is affected by the intermolecular bonding that occurs in the ionic compounds.

    \[\text{Solubility} = \dfrac{\text{grams of solute}}{\text{100 grams of solvent}}\nonumber\]

    Figure \(\PageIndex{1}\). Solubility curve of ionic compounds.2

    A saturated solution contains the maximum amount of solute for a given amount of solvent at a constant temperature. A saturated solution is in a state of dynamic equilibrium; no additional solute will dissolve if added to the solvent. An unsaturated solution contains less solute than a saturated solution; therefore it has the capacity to dissolve more solute. A supersaturated solution contains more solute than it can theoretically hold at a given temperature. A supersaturated solution is made by adding additional solute and heating the solution and slowly cooling undisturbed. Sometimes in order for crystallization to occur, a seed crystal must be introduced and the crystallization process is quite fast and dramatic.

    Figure \(\PageIndex{2}\). Crystallization of a supersaturated solution after the addition of a seed crystal.3

    The solubility of most solid and liquid solutions increases as the temperature increases. This is not true for gases; rather the solubility of gases decreases as the temperature increases. The reason for the decrease in solubility is related to the movement of the molecules. Temperature is a measure of the average kinetic energy of the molecules of a substance. An increase in temperature results in an increase in the kinetic energy of the molecules and an increase in thermal energy. The higher kinetic energy causes more motion in the molecules, which causes the intermolecular attraction to decrease and the gas molecules of solute can escape the solution.1 This can easily be seen when an open can of soda in placed at room temperature or in a cool refrigerator. If an individual would taste the two cans, of soda the can at room temperature would taste very “flat” since more of the “fizz” which is carbon dioxide has escaped the can. Boiled water also tastes “flat” because more of the oxygen gas has been removed.

    Figure \(\PageIndex{3}\). A graph of the solubility of gases versus temperature, showing that as the temperature of the solution increases the solubility of a gas decreases.4

    Thermal pollution occurs when the temperature of a natural body of water, such as, river, lake or ocean is altered. This alters the natural environment and impacts the organisms that live or rely on an aquatic ecosystem. Thermal pollution is associated with electrical power plants and industrial factories that that use fossil fuels or nuclear power; this results in the release of large amounts of excess thermal energy.5 Water absorbs the energy and is converted to steam, which is used to turn turbines and generate electricity. As some point, the water will need to be dumped back into nearby river, lake, or ocean and if not properly cooled, it can increase the temperature of the natural body of water. Thermal pollution can also involve putting colder the normal water into a nearby body of water.

    Elevated water temperatures can also arise from deforestation and urbanization which causes changes in the landscape surrounding a body of water. When trees surrounding a body of water are removed, the now unshaded water can increase its temperature by as much as 10 oC. Thermal pollution can even result when the landscape away from the body of water is altered. These changes in landscape may cause erosion, changes in water volume, and pollutants that may change the composition of the water. For example, muddy water absorbs more energy from the sun than clear water.

    Every response of an aquatic organism from incubation of the egg, to feeding activity, digestive and metabolic processes, reproduction, geographic distribution, and even survival occurs due to the thermal range by their immediate environment. The solubility of gases in water plays a major role in the survival of ectotherms, such as fish. Since an increase in temperature contributes to a decline in the solubility of gases in water; water that is of a higher temperature lacks oxygen. Therefore, many fish are only capable of existing in waters of a lower temperature where ample oxygen exists. Another problem that may occur due to thermal pollution is a limited metabolic range of many aquatic organisms. Many living organisms have metabolic enzymes that control the oxidation of organic food matter. Metabolic enzymes can only function in a narrow range of temperatures, and most enzymes show an optimum temperature in which they reach maximum rate of catalytic activity.5 Small changes in temperature can prove to be deadly for fish. Thermal pollution can also decrease the biodiversity of an ecosystem and allow for the creation of environment that can be invaded by alien aquatic species that may disrupt or destroy an entire ecosystem.5

    Sea turtles in the San Diego Bay are example of how temperature may have affected the metabolic rate of an organism. In 1956, an electric power plant was built near the San Diego Bay, and is thought to be the reason for a colony of East Pacific green turtles living in the Bay. These turtles chose this location as their habitat; in spite of the fact, that it is one of the busiest harbors in the world. The power plant discharges the water that is 15 degrees above the normal temperature of the water in the Bay.6 Researchers think that the warm water is the attraction for the turtles but due a decrease in oxygen in the water, the metabolic rate of the turtles is increased. The Eastern Pacific green turtles in this region are supersized, with some weighing as much as 550 pounds.6 The sizes of these turtles are unprecedented in their enormity and are almost the double the normal size records of the same species in other habitats.6 Carl Mayhugli, a Prescott College graduate, is working on his Masters on the movements and habitats of these turtles. He theorizes that the turtles are able to reach such massive size because the warmer water allows them to continue to forage for food when most sea turtles due to cooler water enter a state of hibernation called torpor.6 The warm water allows eelgrass and algae to grow which provides a supplement to the turtle’s normal diet of tube worms, sea hares, jellyfish and other slow-moving soft-bodied sea creatures.6 The massive turtles are able to increase their food intake by increasing their herbivory diet. The power plant is set to be decommissioned in the next several years but the hope is they will continue to keep the bay as their nesting location.

    From ChemPRIME: 10.15: Saturated and Supersaturated Solutions

    References

    1. Brown, T. Lemay, H., and Bursten, B. Chemistry the Central Science, 10th ed.; Pearson Education, Inc.: Upper Saddle River, NJ, 2006.

    2. Solubility Curve image. www.chem.ufl.edu/~itl/2045/lectures/lec_i.html (accessed July 13, 2011).

    3. Crystallization image. www.chem.ufl.edu/~itl/2045/lectures/lec_i.html (accessed July 13, 2011).

    4. Solubility of Gases image. webserver.dmt.upm.es/~isidoro.../Exercise2.htm (accessed July 13, 2011).

    5. EoEarth Organization. http://www.eoearth.org/article/Thermal_pollution?topic=49471(accessed July 13, 2011).

    6. Bahnsen, C. “Saving the Green Giants” The Environmental Magazine 2009, August 31, 14-16.

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