C6H6: 29

parts. The large latent heat of freezing of liquid water imposes a requirement for large transport of heat from bodies of water before the temperature can drop very much below 0CC, yet another type of thermostat.

A further unusual property of water is that it has a maximum density at around 4°C and expands upon freezing, again because of hydrogen bonds. There are more of these bonds in ice than in liquid water, creating a relatively open crystal structure in the solid phase. When ice melts (requiring the addition of a large amount of latent heat of freezing or fusion, Lf) some of the hydrogen bonds are broken, and a tighter packing of H20 molecule results in the denser liquid.

The high latent heat of evaporation or vaporization - due to the hydrogen bonds causing attraction of water molecules to each other - also causes molecules at the surface of water to have cohesive forces. This results in water having anomalously high surface tension. In turn, this property plays a very strong role in the process of nucleation of cloud droplets, as one of the key factors involved in determining cloud droplet sizes and growth and coalescence rates. The latter is a significant factor in delivery of water to the continents by rain.

Very short wavelengths (A < 186 nm) of UV radiation are required to dissociate the very strong O-H in water (bond strength = 456 kj/ mol). The large concentrations of 02 and 03 in air absorb incoming solar UV radiation high in the atmosphere, preventing much of this photodissociation. The strong bonds and the small size and mass of H20 also give it a very complex infrared absorption spectrum that extends to shorter wavelengths (i.e. higher frequencies, v, and higher energies, hv) than many other simple molecules (C02 for example). One absorption feature, the 6.3 /<m band, is extremely strong, as can be seen in Fig. 6-2. The result of this strong IR absorption, the large amount of water vapor in the atmosphere, and the proximity of the 6.3 /;m absorption band of water vapor to the peak of the Earth's black-body emission is that water vapor is by far the dominant greenhouse gas.

We can see the importance of water vapor as a greenhouse gas by comparing the greenhouse effect on Earth, a relatively humid planet, with

Fig. 6-2 Comparison of infrared absorbance of a vertical column of atmospheric C02 and H20 vapor. The nearly total absorbance by HzO between 5 and 7 /¡m, nearly coinciding with the peak of the wavelength-dependent emission of the surface, make H20 a much more effective greenhouse gas. Liquid water (not shown) in clouds adds still more absorbance.

Fig. 6-2 Comparison of infrared absorbance of a vertical column of atmospheric C02 and H20 vapor. The nearly total absorbance by HzO between 5 and 7 /¡m, nearly coinciding with the peak of the wavelength-dependent emission of the surface, make H20 a much more effective greenhouse gas. Liquid water (not shown) in clouds adds still more absorbance.

that on Mars, which is arid. Mars has an atmosphere that is ca. 95% C02 (by mass, about 50 times more than on Earth). The greenhouse effect of Martian C02 causes a temperature increase of only a few degrees. In contrast, the total natural greenhouse effect on Earth is ca. 33 K, the majority of which is due to water vapor and water clouds. Nonetheless, as will be seen in Chapter 17, the anthropogenic greenhouse effect due to enhanced C02, CH4, N20 etc. cannot be dismissed, as changes in the Earth's average temperature of even 1 K are significant.

Besides these special physical properties, hydrogen-bonded liquid water also has unique solvent and solution properties. One feature is high proton (H+) mobility due to the ability of individual hydrogen nuclei to jump from one water molecule to the next. Recalling that at temperatures of about 300 K, the molar concentration in pure water of H30+ ions is ca. 10 ~ 7 M, the "extra" proton can come from either of two water molecules. This freedom of H+ to transfer from one to an adjacent "parent" molecule allows relatively high electrical conductivity. A proton added at one point in an aqueous solution causes a domino effect, because the initiating proton has only a short distance to travel to cause one to pop out somewhere else.

The existence of strongly polar water molecules and mobile protons also makes H20 an excellent and almost universal solvent for ionic compounds and polar organic species. Compounds that will not significantly dissolve in water (i.e. saturated solutions with concentrations less than ca. 10"5 M) include aliphatic and aromatic hydrocarbons, as well as plastics and many other polymers.

6.1.2 The Right Abundance of Water to Support Life

As can be seen in Fig. 2-1 (abundance of elements), hydrogen and oxygen (along with carbon, magnesium, silicon, sulfur, and iron) are particularly abundant in the solar system, probably because the common isotopic forms of the latter six elements have nuclear masses that are multiples of the helium (He) nucleus. Oxygen is present in the Earth's crust in an abundance that exceeds the amount required to form oxides of silicon, sulfur, and iron in the crust; the excess oxygen occurs mostly as the volatiles C02 and H20. The C02 now resides primarily in carbonate rocks whereas the H20 is almost all in the oceans.

While it is clear that the hydrosphere is a significant portion of the planet's mass, there is not, at least currently, so much water that the continents are submerged. Conversely, the oceans are large enough that their surface area would never become an important limiting factor in the hydrologic cycle. Although there have been many shifts in the balance of the hydrosphere, this condition has prevailed since the biosphere began to evolve. The presence of liquid water allowed the Earth to become and remain a living planet, and by the astronomical coincidence of the Earth's location, the planet received just the proper abundance to sustain and recycle this all-important resource, almost in perpetuity.

6.2 Global Water Balance

While the hydrosphere has long been appreciated as essential to life on Earth, only in the past couple of decades have scientists expanded their exploration of the global hydrologic cycle and its roles across the spectrum of Earth science disciplines. The Earth and its atmosphere, in the broadest view, are a complex, intimately coupled system of chemical, physical, and biological cycles, and water, with its myriad unique chemical and physical properties, plays a part in almost all of them.

To understand the role water plays in global cycles, it is necessary to first understand the mechanics of the water cycle. The hydrologic cycle is driven by solar radiation, which provides the energy necessary to overcome latent heat capacities involved in phase changes. Gravity plays a key role in returning condensed water to the surface as precipitation, and via runoff from the continents to the oceans. At the simplest level, the global water cycle results from imbalances between precipitation and évapotranspiration (ET) at the ocean and land surfaces. Globally, the oceans lose more water by evaporation than they gain by precipitation, whereas the land surface receives more precipitation than is lost through ET; runoff from the land surfaces then balances the ocean-atmosphere water deficit. The hydrologic cycle is significantly more complex than this simple description would suggest. In addition to the atmosphere, oceans, and rivers, significant amounts of water are stored in groundwater, glaciers and ice sheets, soil moisture, and, to a smaller extent, biomass. Figure 6-3 shows a schematic of the global hydrologic cycle, with storages in km3 and fluxes in km3/yr.

Fig. 6-3 Global water balance. Storages in km3, fluxes in km3/yr. Turnover times calculated as storage divided by total annual inflow. (Data from Shiklomanov and Sokolov, 1983.)
Table 6-2 Reservoir storage


UNESCO (1978) (millions of km3)

Nace (1969) (millions of km3)

Baumgartner and Reichel (1975) (millions of km3)


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