It is hard to imagine any part of the Earth system that is more essential than or that has as many different functions as the water of the hydrosphere. In particular, the presence of a mobile liquid phase, with its long list of special chemical and physical properties, must be clearly identified as the main feature of Earth that separates it from the other terrestrial planets or from any known astronomical object. Close to home, the "terrestrial planets," Earth, Mars, and Venus are presumed to have accreted similar abundances of "excess volatiles" - H20, C02, etc. - but evolved very differently. Even in the earliest stages of planetary evolution, liquid water provided a medium in which chemical reactions occurred between atmospheric C02 and the minerals in primitive igneous rocks to allow the precipitation of carbonate minerals and to prevent a runaway greenhouse effect. While no exact chronology or quantification of this early chemical event can be given, it seems clear that some such process prevented the accumulation of all of the Earth's C02 and H20 (as a vapor) in the atmosphere at the same time. This would have caused the Earth to be moie or less like Venus - a condition from which there would appear to be no return to our present state. Before embarking on a description of this most important reservoir, it is useful - perhaps necessary - to reflect on the special properties of water itself. We can then proceed to a discussion of how the hydrosphere works.
The water molecule, H20, structurally is a bent molecule with a very strong permanent dipole moment. This dipole is the result of the negatively charged O atom and the two positively charged H atoms (the whole molecule being neutral). The existence of this charge separation arises due to the near orthogonality of the orbitals of the bonding electrons of the central O atom, while its large magnitude arises comes from the lack of shielding of the bonding electron and the small size of the O atom.
The permanent dipole moment is so strong that it permits the function of what are called hydrogen bonds between the highly electronegative O atom of one molecule and a nearby hydrogen atom of another molecule (see Fig. 61). The hydrogen bond is not a chemical bond in the ordinary sense of the forces that hold molecules together, which can be deduced from its strength of ca. 20 kj/mol. Ordinary molecular bonds have typical strengths (energy required to break them) of a few hundred kj/mol.
It is the hydrogen bonds of water that give it unique physical and chemical properties, characteristics that set it apart from all of the other molecules formed from elements near the top of the periodic table. Table 6-1 compares several key properties of water to selected
Earth System Science ISBN 0-12-379370-X
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Fig. 6-1 Hydrogen bonds in liquid water.
simple compounds that might be expected to have similar properties to water but do not. In regard to melting and boiling point, water behaves like a much larger molecule, but it has low density like the low atomic number compound that it really is.
Likewise, liquid water has anomalously high molar heat capacity (75 J/mol K), meaning that liquid water can absorb relatively large amounts of heat from the sun by day and release it at night without much change of temperature. Owing to the large amount of liquid water at the surface of Earth, this large heat capacity is important in mediating temperatures and therefore climate.
Still further, water has large latent heats of evaporation and freezing (J/mol), all because of the same hydrogen bonds. As one result, the solar heating of the planet (largely in the tropics and subtropics), which results primarily in evaporation, transfers latent heat to the atmosphere in the form of water vapor. Subsequent precipitation at colder temperatures (higher latitudes or altitudes) releases the latent heat, making water vapor an important heat-transport vehicle.
This latent heat of evaporation, Le, also appears in the fundamental description of the dependence of the vapor pressure of water, p, on temperature, T - the Clausius-Clapeyron equation:
or in integral form, between locations with p\, T¡ and p2, T2:
where R is the universal gas constant in appropriate units.
The large value of Le results in a very strong dependence of vapor pressure on temperature. As a result, the water vapor content of the air is extremely variable, from parts per million by volume in the coldest parts of the atmosphere to several percent in the warmest and wettest
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