CHEMISTRY THE CENTRAL SCIENCE

18 CHEMISTRY OF THE ENVIRONMENT

18.3 EARTH'S WATER

Water covers 72% of Earth's surface and is essential to life. Our bodies are about 65% water by mass. Because of extensive hydrogen bonding, water has unusually high melting and boiling points and a high heat capacity. (Section 11.2) Water's highly polar character is responsible for its exceptional ability to dissolve a wide range of ionic and polar-covalent substances. Many reactions occur in water, including reactions in which H₂O itself is a reactant. Recall, for example, that H₂O can participate in acid–base reactions as either a proton donor or a proton acceptor. (Section 16.3) All these properties play a role in our environment.

The Global Water Cycle

All the water on Earth is connected in a global water cycle (FIGURE 18.15). Most of the processes depicted here rely on the phase changes of water. For instance, warmed by the Sun, liquid water in the oceans evaporates into the atmosphere as water vapor and condenses into liquid water droplets that we see as clouds. Water droplets in the clouds can crystallize to ice, which can precipitate as hail or snow. Once on the ground, the hail or snow melts to liquid water, which soaks into the ground. If conditions are right, it is also possible for ice on the ground to sublime to water vapor in the atmosphere.

GIVE IT SOME THOUGHT

Consider the phase diagram for water shown in Figure 11.28 (page 446). In what pressure range and in what temperature range must H₂O exist in order for H₂O(s) to sublime to H₂O(g)?

FIGURE 18.15 The global water cycle.

Salt Water: Earth's Oceans and Seas

The vast layer of salty water that covers so much of the planet is in actuality one large connected body and is generally constant in composition. For this reason, oceanogra-phers speak of a world ocean rather than of the separate oceans we learn about in geography books.

The world ocean is huge, having a volume of 1.35 × 10⁹ km³ and containing 97.2% of all the water on Earth. Of the remaining 2.8%, 2.1% is in the form of ice caps and glaciers. All the freshwater—in lakes, in rivers, and in the ground—amounts to only 0.6%. Most of the remaining 0.1% is in brackish (salty) water, such as that in the Great Salt Lake in Utah.

Seawater is often referred to as saline water. The salinity of seawater is the mass in grams of dry salts present in 1 kg of seawater. In the world ocean, salinity averages about 35. To put it another way, seawater contains about 3.5% dissolved salts by mass. The list of elements present in seawater is very long. Most, however, are present only in very low concentrations. TABLE 18.5 lists the 11 ionic species most abundant in seawater.

TABLE 18.5 • Ionic Constituents of Seawater Present in Concentrations Greater Than 0.001 g/kg (1 ppm)

GO FIGURE

Look at the trend in density as a function of depth; does it mirror the trend in salinity or in temperature?

FIGURE 18.16 Average temperature, salinity, and density of seawater as a function of depth. (From Windows to the Universe, University Corporation for Atmospheric Research. Copyright © 2004 University Corporation for Atmospheric Research. All rights reserved.)

Seawater temperature, salinity, and density vary as a function of depth (FIGURE 18.16). Sunlight penetrates well only 200 m into the water; the region between 200 m and 1000 m deep is the “twilight zone,” where visible light is faint. Below 1000 m, the ocean is pitch-black and cold, about 4 °C. The transport of heat, salt, and other chemicals throughout the ocean is influenced by these changes in the physical properties of seawater, and in turn the changes in the way heat and substances are transported affects ocean currents and the global climate.

The sea is so vast that if the concentration of a substance in seawater is 1 part per billion (1 × 10^–6 g per kilogram of water), there is 1 × 10¹² kg of the substance in the world ocean. Nevertheless, because of high extracting costs, only three substances are obtained from seawater in commercially important amounts: sodium chloride, bromine (from bromide salts), and magnesium (from its salts).

Absorption of CO₂ by the ocean plays a large role in global climate. Because carbon dioxide and water form carbonic acid, the H₂CO₃ concentration in the ocean increases as the water absorbs atmospheric CO₂. Most of the carbon in the ocean, however, is in the form of HCO₃^– and CO₃^2– ions, which form a buffer system that maintains the ocean's pH between 8.0 and 8.3. The pH of the ocean is predicted to decrease as the concentration of CO₂ in the atmosphere increases, as discussed in the “Chemistry and Life” box on ocean acidification in Section 17.5.

Freshwater and Groundwater

Freshwater is the term used to denote 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. The United States is fortunate in its abundance of freshwater—1.7× 10¹⁵ L (660 trillion gallons) is the estimated reserve, which is renewed by rainfall. An estimated 9 × 10¹¹ L of freshwater is used every day in the United States. Most of this is used for agriculture (41%) and hydroelectric power (39%), with small amounts for industry (6%), household needs (6%), and drinking water (1%). An adult drinks about 2 L of water per day. In the United States, our daily use of water per person far exceeds this subsistence level, amounting to an average of about 300 L/day for personal consumption and hygiene. We use about 8 L/person for cooking and drinking, about 120 L/person for cleaning (bathing, laundering, and houseclean-ing), 80 L/person for flushing toilets, and 80 L/person for watering lawns.

GO FIGURE

What factors influence how long it takes for water to migrate from a deep aquifer to the surface?

FIGURE 18.17 Groundwater is water located in aquifers below the soil. An unconfined aquifer, that has no dense rock between it and the water table, can hold water for days or years. Confined aquifers can hold water for centuries or millenia, depending on their depth. Aquifers are discharged through wells or rivers, and are recharged from water flowing through the soil (e.g., from rain).

The total amount of freshwater on Earth is not a very large fraction of the total water present. Indeed, freshwater is one of our most precious resources. It forms by evaporation from the oceans and the land. The water vapor that accumulates in the atmosphere is transported by global atmospheric circulation, eventually returning to Earth as rain, snow, and other forms of precipitation (Figure 18.15).

As water runs off the land on its way to the oceans, it dissolves a variety of cations (mainly Na⁺, K⁺, Mg²⁺, Ca²⁺, and Fe²⁺), anions (mainly Cl^–, SO₄^2–, and HCO₃^–), and gases (principally O₂, N₂, and CO₂). As we use water, it becomes laden with additional dissolved material, including the wastes of human society. As our population and output of environmental pollutants increase, we find that we must spend ever-increasing amounts of money and resources to guarantee a supply of freshwater.

Approximately 20% of the world's freshwater is under the soil, in the form of groundwater. Groundwater resides in aquifers, which are layers of porous rock that hold water. The water in aquifers can be very pure and accessible for human consumption if near the surface (FIGURE 18.17). Dense rock that does not allow water to readily penetrate can hold groundwater for years or even millennia.

The nature of the rock that contains the groundwater has a large influence on the water's chemical composition. If minerals in the rock are water soluble to some extent, ions can leach out of the rock and remain dissolved in the groundwater. Arsenic in the form of HAsO₄^2–, H₂AsO₄^–, and H₃AsO₃ are found in groundwater across the world, most infamously in Bangladesh, at concentrations poisonous to humans.