Water anomalous density

From this discussion, we would predict that given sufficient cooling, a liqifid should be transformed into a solid, more dense phase. Why, then, does ice float in liquid water Some force must keep the water molecules far enough apart in ice so as to cause its density to be lower than that of liquid water. It is somewhat ironic that the most abundant and important of liquids on our planet is the only one to exhibit this anomalous density behavior. 

The anomalous density behavior of water has important consequences for the survival of aquatic organisms at mid-latitudes. As winter approaches, the surface waters of ponds and lakes cool. The ensuing increase in density causes this water to sink to the bottom of the water body. This process continues until water temperatures drop... 

The temperature at which seawater reaches its maximum density also decreases with increasing salinity. Most seawater in the ocean has a salinity between 33% and 37%. At salinities greater than 26%, the freezing point of seawater is higher than the temperature at which it reaches its maximum density. Thus, seawater never undergoes the anomalous density behavior of pure water. Instead, sea ice floats because it is mostly pure water (some pockets of brine are often occluded into the crystal structure). 

Water as a solvent has several anomalous features (e.g. anomalous density, the only nontoxic and liquid hydride of the nonmetals, melting point varying with pressure, dielectric constant) and with its two- or even three-dimensional structure has still not been fully researched. 

The freezing point diagram for the hydrazine—water system (Eig. 1) shows two low melting eutectics and a compound at 64 wt % hydrazine having a melting point of —51.6°C. The latter corresponds to hydrazine hydrate [7803-57-8] which has a 1 1 molar ratio of hydrazine to water. The anomalous behavior of certain physical properties such as viscosity and density at the hydrate composition indicates that the hydrate exists both in the Hquid as well as in the soHd phase. In the vapor phase, hydrazine hydrate partially dissociates. 

The anomalous increase of the water uptake observed in Fig. 10 when approaching equilibrium at 60 °C has been associated to the damage. The abrupt upturn of the sorption curve may be explained considering a possible crazing of the low crosslinked internodular matrix induced by the differential swelling stresses that can arise, at high water contents, between areas of different crosslinking density. 

The molecular collective behavior of surfactant molecules has been analyzed using the time courses of capillary wave frequency after injection of surfactant aqueous solution onto the liquid-liquid interface [5,8]. Typical power spectra for capillary waves excited at the water-nitrobenzene interface are shown in Fig. 3 (a) without CTAB (cetyltrimethy-lammonium bromide) molecules, and (b) 10 s after the injection of CTAB solution to the water phase [5]. The peak appearing around 10-13 kHz represents the beat frequency, i.e., the capillary wave frequency. The peak of the capillary wave frequency shifts from 12.5 to 10.0kHz on the injection of CTAB solution. This is due to the decrease in interfacial tension caused by the increased number density of surfactant molecules at the interface. Time courses of capillary wave frequency after the injection of different CTAB concentrations into the aqueous phase are reproduced in Fig. 4. An anomalous temporary decrease in capillary wave frequency is observed when the CTAB solution beyond the CMC (critical micelle concentration) was injected. The capillary wave frequency decreases rapidly on injection, and after attaining its minimum value, it increases... 

Water has several anomalous features (e.g., density, being the only nontoxic and liquid "hydride" of the non-metals, melting point varying with pressure, etc.). Of direct importance for the aqueous biphasic process are the physiological (entries 2 and 4 of Table 5.1), economic (1,3,6,9), ecological/safety-related (2,3,4,9), process engineering (1,6,7,9,10,11,12), and chemical and physical properties (1,5,6,8,11,13) of water. The different properties interact and complement each other. Thus water, whose high... 

FIG. 5 The density of liquid and supercooled water as a function of temperature, illustrating the anomalous liquid phase density maximum of water (data from Lide, 2002-2003). 

Ludwig s (2001) review discusses water clusters and water cluster models. One of the water clusters discussed by Ludwig is the icosahedral cluster developed by Chaplin (1999). A fluctuating network of water molecules, with local icosahedral symmetry, was proposed by Chaplin (1999) it contains, when complete, 280 fully hydrogen-bonded water molecules. This structure allows explanation of a number of the anomalous properties of water, including its temperature-density and pressure-viscosity behaviors, the radial distribution pattern, the change in water properties on supercooling, and the solvation properties of ions, hydrophobic molecules, carbohydrates, and macromolecules (Chaplin, 1999, 2001, 2004). 

The conditions on the phase diagram for which this anomalous behavior occurs has been termed water s structurally anomalous region. Inspection of the order map (Figure 4) reveals a dome of structural anomalies within the temperature-density plane, bounded by loci of maximum tetrahedral order (at low densities) and minimum translational order (at high densities) as shown in Figure 5. Also marked on Figure 5 are regions of diffusive anomalies,... 

Figure 5 Relationship among loci of structural, dynamic, and thermodynamic anomalies in SPC/E water. The structurally anomalous region is bounded by the loci of q maxima (upward-pointing triangles) and t minima (downward-pointing triangles). Inside of this region, water becomes more disordered when compressed. The loci of diffusivity minima (circles) and maxima (diamonds) define the region of dynamic anomalies, where self-diffusivity increases with density. Inside of the thermodynamically anomalous region (squares), the density increases when water is heated at constant pressure. Reprinted with permission from Ref. 29.

An attempt has been made to decide between these, two hypotheses advanced to explain the anomalous values obtained for density of charcoals by examining the density temperature curve for a system comprising water and charcoal. From a knowledge of... 

Equations (7.31a, b) imply that the melting curve slopes forward in the usual case (e.g., Fig. 2.9 for C02) where the density of the solid exceeds that of the liquid (psoiid > PiiquidX but this curve tilts backward in the anomalous case of water (Fig. 7.1), where psoiid < Piiquid (ice cubes float ). The relationships (7.31a, b) can also be paraphrased and generalized to any phase transition a in the statement... 

The chloride, NO Cl, behaves anomalously upon hydrolysis the two oxygen atoms exert a strong electron pull. The electron density at the chlorine atom is thus sufficiently reduced so that attack by water occurs as shown, and hypochlorous acid and nitrous acid result ... 

A typical temperature scan of a system after prolonged isothermal cure is shown in Fig. 5. In comparing post-cure behavior with that after cure at Te , note the increase in Tg as well as in the temperature of the secondary transition (TgeJ. Also note that the relative rigidity (modulus) of the post-cured material is lower at room temperature (RT) than that of the partially-cured specimen. This behavior is anomalous, because post-cure would be expected to increase the crosslinking, and hence the stiffness of the material. The lower modulus manifests itself in a lower density and greater water absorption at RT for the more highly cured material than for the partially-cured... 

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