Water is a colorless, odorless and tasteless liquid, stable and nontoxic. Its solid and vapor forms are ice and steam, although the term "water" is sometimes used for all three phases. With a narrow liquid range, it is one of the few chemicals known to exist commonly in all three phases (Figure 1).
Figure 1. PVT Surface for water From Eisenberg, D. and Kauzmann, W. (1969) The Structure and Properties of Water, by permission of Oxford University Press.
Water is a relatively "light" molecule with a molar mass (molecular weight) of 18.01, giving the vapor a density of 0.623 relative to air. The high dielectric constant (Table 1) of water is largely responsible for its good solvent properties. It is, however, a weak electrolyte when pure, but with dissolved ions increases its conductivity by several orders of magnitude. The molecule's small size (O-H bond length is 96 pm) gives high permeability through membranes (see Membrane Processes). In many ways water is an anomalous fluid largely explained by strong hydrogen bonding resulting in the formation of loose clusters of molecules [see also Franks (1972)]. It has a high latent heat (see Steam Tables) and high melting point, and expands on freezing. Above the critical point (Table 1) water is "supercritical" and the distinction between liquid and vapor disappears.
From Fundamentals of Heat and Mass Transfer, Incropera, F. P. and de Witt, D. O. (1990) Wiley.
Figure 2. Surface tension σ, mN/m and Prandtl Number, Pr, of saturated water. [Incropera & de Witt (1990)].
Water forms hydrates with many salts, e.g., CaSO4.2H2O (gypsum) and CaSO41/2H2O which decompose as temperature is increased, e.g., . In the case of temperature rises, through loss of "water of crystallization", and so scaling of heated surfaces is often more severe with waters containing calcium sulfate.
Water forms hydrates with carbon dioxide and the lower aikanes (CH4, C2H6) at high pressure (>5 MPa) and temperatures typically below 10°C [see Berecz and Balla-Achs (1983)]. It is reactive with many metals forming basic oxides (e.g., Zn → ZnO) or hydroxides (e.g., Na → NaOH, Ca → CaO → Ca(OH)2), and with nonmetallic oxides giving acids (e.g., SO3 → H2SO4). Water acts as a catalyst in some reactions and affects the activity of enzymatic catalysts (e.g., porcine lipase has a maximum activity at about 10-12% water).
Berecz, E. and Balla-Achs, M. (1983) Gas Hydrates, Elsevier.
Eisenberg, D. & Kauzmann, W. (1969) The Structure and Properties of Water, Oxford, Clarendon.
Franks, E, Ed., Water, A Comprehensive Treatise, 1972-c. 1982 Vol. 1-7, Plenum, NY.
Horvath, A. L., (1975) Physical Properties of Inorganic Compounds, Arnold.
Incropera, F. P. and de Witt D. P. (1990) Fundamentals of Heat and Mass Transfer, 3rd edn., Wiley.
Perry, R. H. and Green, D. ed. (1985) Perry's Chemical Engineers' Handbook, 6th edn., McGraw-Hill.