Physical properties of water

How this vast and enduring body of water reacts to the forces placed upon it is related to the somewhat anomalous physical properties of water itself. Given its low molecular weight (18 dal-tons), water is a relatively dense, viscous and barely compressible fluid (see Table 2.1 for reference), with relatively high melting and boiling points. This behaviour is due to the asymmetry of the water molecule and to the fact that the two hydrogen atoms, each sharing its electron with the oxygen atom, are held at a relatively narrow angle on one side of the molecule. In turn, this gives a polarity to the molecule, one side (the 'hydrogen side') having a net positive charge and the other (the 'oxygen side') a net negative one. The molecules then have a mutual attraction, giving rise to the formation of aquo polymers. It is the complexation into larger molecules which raises the melting point of what is otherwise a low-molecular-weight compound into

Figure 2.1

Plots showing (a) the density and (b) the absolute viscosity of pure water as a function of temperature. Redrawn, with permission, from Reynolds (1997a).

Figure 2.1

Plots showing (a) the density and (b) the absolute viscosity of pure water as a function of temperature. Redrawn, with permission, from Reynolds (1997a).

the range we perceive as being normal. As temperature is raised and the motion of molecules is increased, individual molecules break from the complexes. In most liquids, the molecules come to occupy more space, that is the liquid expands and the density decreases. In water, this effect is countered by the fact that the liberated molecules fall within the complexes, so that the same number of molecules occupies less space, leading to increased density. In pure water, the latter effect dominates up to 3.98 °C; above this temperature, the separation of molecules becomes, progressively, the dominant effect and the liquid expands accordingly (see Fig. 2.1).

The molecular behaviour explains not only why fresh water achieves its greatest density at close to 4 °C but also why, under appropriate conditions, ice forms at the surface of a lake (where, incidentally, it insulates the deeper water against further heat loss to the atmosphere), and why, with every degree step above 4 ° C, the difference in density also becomes greater. Limnologists are well aware of the effect this has in enhancing the mechanical-energy requirement to mix increasingly warmed surface waters with the dense layers below; the limnetic ecologist is familiar with the impact of both processes on the environments of phytoplankton.

The same principles apply in the sea and in salt lakes, except that the higher concentrations of dissolved ionic salts, their separation into constituent charged ions and their attraction to the opposite-charged poles of the water molecules all contribute further modifications to the polymerisation. Individual ions become surrounded by water molecules in a hydrated layer, disrupting their structure and altering the properties of the liquid. The salinity of sea water ranges from a trace (in some estuaries and adjacent to melting glaciers) to a maximum of about 40 gkg-1 (the Red Sea; note that this is greatly exceeded in some inland lakes). In most of the open ocean, salinity is generally about 35 (±3) gkg-1, having a density of 27 (±2) kgm-3 greater than pure water of the same temperature. The presence of salt depresses not just the freezing point but also the temperature of maximum density. When the salt content is about 25 gkg-1, these temperatures coincide, at -1.3 °C. Thus, in most of the sea, the density of water increases with lowering temperatures right down to freezing. Sea ice does not form at the surface, as does lake ice, simply as a consequence of cooling of the water. Normally, some other component (dilution by rain and or terrestrial run-off) is necessary to decrease the density of the topmost water.

Molecular behaviour influences the temperature-dependent viscous properties of water. Viscosity, manifest as the resistance provided to one water layer to the slippage of another across it, decreases rapidly with rising temperature (Fig. 2.1b); according to the standard definition of viscosity, this means there is a decreasing resistance of one water layer to the slippage of another across it, for the same given difference in temperature. Viscosity is greater in sea water than in pure water: an increment about 0.1 x 10-3 kgm-1s-1 applies to water containing 35 gkg-1 over a normal temperature range. High viscosity, like large differences in density, is an effective deterrent to physical mixing and mechanical heat transfer.

0 0

Post a comment