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18 Chemistry of the Environment Solutions to Exercises 18.5 The Sun 18.6 Saltwater, freshwater and groundwater differ in salt content, location and percentage of Earth's total water. Saltwater contains high concentrations of dissolved salts and solids and includes the world ocean (97.2% of all water) and brackish or salty water (0.1% of all water) in places such as the Great Salt Lake and the Chesapeake Bay. The world ocean averages about 35 g of dissolved salts per kg of water, or 35,000 ppm. Freshwater (0.6% of all water on earth) refers to natural waters that have low concentrations (less than 500 ppm) of dissolved salts and solids. Freshwater includes the waters of lakes, rivers, ponds, and streams. Groundwater is freshwater that is under the soil. It resides in aquifers, porous rock that holds water, and composes 20% of the world's freshwater. 18.7 dissolves in seawater to form The basic pH of the ocean encourages ionization of fo form (aq) and Under the correct conditions, carbon is removed from the ocean as CaCO₃(s) (sea shells, coral, chalk cliffs). As carbon is removed, more dissolves to maintain the balance of complex and interacting acid-base and precipitation equilibria. 18.8 Analyze/Plan. Explain how an ion-exchange column "softens" water. See the Closer Look box on "Water Softening" in Section 18.4. Solve. The plastic beads in an ion-exchange column contain covalently bound anionic groups such as R-COO- and These groups have Na+ cations associated with them for charge balance. When "hard" water containing Ca²⁺ and other divalent cations passes over the beads, the 2+ cations are attracted to the anionic groups and Na+ is displaced. The higher charge on the divalent cations leads to greater electrostatic attractions, which promote the cation exchange. The "soft" water that comes out of the column contains two Na+ ions in place of each divalent cation, mostly Ca²⁺ and that remains in the column associated with the plastic beads. 18.9 The guiding principle of green chemistry is that "an ounce of prevention is worth a pound of cure." Processes should be designed to minimize or eliminate solvents and waste, generate nontoxic waste, be energy efficient, employ renewable starting materials, and take advantage of catalysts that enable the use of safe and common reagents. 18.10 Some of the missing is absorbed by "land plants" (vegetation other than trees) and incorporated into the soil. Soil is the largest land-based carbon reservoir. The amount of carbon-storing capacity of soil is affected by erosion, soil fertility, and other complex factors. For more details, search the internet for "carbon budget." Earth's Atmosphere (section 18.1) 18.11 (a) The temperature profile of the atmosphere (Figure 18.1) is the basis of its division into regions. The center of each peak or trough in the temperature profile corresponds to a new region. 552