A few properties of the hydrosphere, including its interaction with the other spheres, were described in Section 3.2. In this chapter we present fundamental aspects of the hydrosphere's chemistry that are critical for a thorough understanding of the environment. Pollution of water and its treatment are discussed in Section 9.5.

Chemistry in an aqueous solution always takes place with a great surplus of water. At room temperature, the amount concentration of water is caq ≈ 55.5 m, which may be compared with cNaCl ≈ 0.5 m in seawater (see Table 3.13). Seawater is a fairly concentrated solution, and even here each ion is surrounded by about 100 water molecules. Therefore, the physicochemical properties of water itself underlie all chemistry in the hydrosphere. We shall see that water exerts a leveling effect, not only on the apparent strengths of acids and bases, but also on the range of electrode potentials that are observed, allowing species to be stable in solution. In this chapter we discuss acid-base chemistry, coordination chemistry, redox chemistry, and the thermodynamics of species in aqueous solution.