Water

Key point: Hydrogen bonding in water results in a high boiling liquid and a highly structured arrangement in the solid, ice. At least nine distinct forms of ice have been identified. At 0C and atmospheric pressure, hexagonal ice I_h forms (Fig. 10.7) but between 120 and 140C the cubic form, Ic , is produced. At very high pressures several higher density polymorphs are formed,

Figure. 16.2 Frost diagram for the elements of Group 16 in acidic solution. The species with oxidation number -2 are H2 E. For oxidation number -1 the compound is H₂O₂. The positive oxidation numbers refer to the oxo acids or oxoanions. some of which are based on silica-like structures (Section 14.10). Water is formed by the direct interaction of the elements: 

This reaction is very exothermic and provides the basis for the development of the hydrogen economy and hydrogen fuel cells (Fig. 5.1, Box 10.2, and Section 24.1). Water is the most widely used solvent not only because it is so widely available but also because of its high relative permittivity (dielectric constant), wide liquid range, and—through a combination of its polar character and ability to form hydrogen bonds—solvating ability. Many anhydrous and hydrated com pounds dissolve in water to give hydrated cations and anions. Some predominantly covalent compounds, such as ethanol and ethanoic (acetic) acid, are soluble in water or miscible with it because of hydrogen-bonded interactions with the solvent. Many other covalent compounds react with water in hydrolysis reactions; examples are discussed in the appropriate chapters. In addition to simple dissolution and hydrolysis reactions, the importance of aqueous solution chemistry can be seen in redox reactions and acid base reactions. Water also acts as a Lewis base ligand in metal complexes (Section 7.1). The deprotonated forms, OH- and particularly the oxide ion O^2- are important ligands for stabilizing higher oxidation states, examples of which are found in the simple oxo-cations of early d-block elements, such as the vanadyl ion, VO⁺².

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مواضيع ذات صلة

Hydrogen peroxide

Selenium, tellurium, and polonium

Sulfur

Reactivity of oxygen

Oxygen

Condensed phosphates

Oxoanions of phosphorus, arsenic, antimony, and bismuth

Oxides of phosphorus, arsenic, antimony, and bismuth

Nitrogen(V) oxides and oxoanions

Oxides and oxoanions of nitrogen

Oxohalides

Halides of the heavy elements