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Consult the TEXTS FOR SUPPLEMENTARY READING and learn about the advent of municipal water treatment (Text 48). Be ready to discuss the information you have read.
38. Read the text to learn about producing fresh water by freezing salt water:
Text 12 D
Freezing Salt Water to Produce Fresh Water
Boiling isn’t the only change of phase that’s used to purify water. The ice that forms when sea water freezes is essentially pure fresh water, a phenomenon that Eskimos have used to obtain potable water for thousands of years. This purification effect results from a balance between energy and disorder. Physical systems tend to minimize their potential energies. For example, a ball tends to roll down hill to minimize its gravitational potential energy. Similarly, a mixture of ice and water tends to minimize its potential energy by excluding contaminants from the ice and leaving them in the water.
Water is disordered already and having solids, liquids, or gases dissolved in it isn’t a problem. Ice, on the other hand, is a highly ordered crystal that is seriously disrupted by the presence of dissolved chemicals. The water molecules in an ice crystal pack more neatly and obtain a lower overall potential energy when the ice contains no contaminants.
So a contaminated mixture of water and ice reaches its lowest overall potential energy when all of the contaminants stay in the water and the ice contains only water molecules. As long as the water freezes slowly, the ice crystal it forms will have very little contamination in it and the unfrozen water will end up with a relatively high concentration of contaminants.
By separating pure from impure water, this process reduces the water’s disorder so that, to avoid violating the second law of thermodynamics, additional disorder must be created somewhere else.
This additional disorder is created in the low temperature region that freezes the salt water. As the salt water freezes, which it does well below water’s ordinary freezing temperature, the salt water releases heat to the low temperature region and introduces considerable disorder into that region. This rise in the low temperature region’s entropy more than makes up for any decrease in entropy in the water as it freezes.
However, disorder appears whenever it can and so the ice crystals that form always contain imperfections. Even if there are no impurities around, the crystals will probably have minor defects. These defects include flaws in the stacking of molecules or empty spots in the otherwise orderly arrays of molecules. Truly perfect crystals are extremely hard or even impossible to grow.
When you freeze a bucket of salt water, the ice that forms first contains very little salt. The salt is in the remaining salt water, which becomes more and more concentrated as additional water molecules are bound up in the ice. By the time there is only a small amount of water remaining, that salt water has a very high concentration of salt and salt crystals begin to form. These salt crystals can easily become trapped in the ice so care must be taken to remove the ice from the concentrated salt solution before salt crystals begin to form. In sea ice, the salt is carried away by the sea water so only pure ice is formed. When you freeze ice cubes, the outer surface freezes first and the impurities become concentrated near the middle of the cube.
One of the main impurities is dissolved air, which eventually comes out of solution and forms tiny air bubbles in the ice. These air bubbles appear as a white cloudy region inside the ice cube. One way to reduce this clouding is to boil the water before you freeze it.
Boiling the water drives most of the air out of solution so that no air bubbles form in the resulting ice. Freezing salt water to form pure ice works best in cold climates where low temperatures are available directly.
Active refrigeration can also freeze salt water to obtain fresh water, but it’s expensive. Because of water’s latent heat of melting, you must remove a large amount of heat from salt water to freeze it. Although refrigerated water desalination plants have been built, they have proven to be less economical than distillation plants.
TEXT AND VOCABULARY EXERCISES
38. Find in the text the words or phrases which mean the same as:
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