Water and Its Phase Changes

OBJECTIVE To learn some of the important features of water. 

The water we drink often has a taste because of the substances dissolved in it. It is not pure water. 

In the world around us we see many solids (soil, rocks, trees, concrete, and so on), and we are immersed in the gases of the atmosphere. But the liquid we most commonly see is water it is virtually everywhere, covering about 70% of the earth s surface. Approximately 97% of the earth s water is found in the oceans, which are actually mixtures of water and huge quantities of dissolved salts. 

Water is one of the most important substances on earth. It is crucial for sustaining the reactions within our bodies that keep us alive, but it also affects our lives in many indirect ways. The oceans help moderate the earth s temperature. Water cools automobile engines and nuclear power plants. Water provides a means of transportation on the earth s surface and acts as a medium for the growth of the myriad aeatures we use as food, and much more. 

Pure water is a colorless, tasteless substance that at 1 atm pressure freezes to form a solid at 0 °C and vaporizes completely to form a gas at 100 °C. This means that (at 1 atm pressure) the liquid range of water occurs between the temperatures 0 °C and 100 °C. 

The heating/cooling curve for water heated or cooled at a constant rate. The plateau at the boiling point is longer than the plateau at the melting point, because it takes almost seven times as much energy (and thus seven times the heating time) to vaporize liquid water as to melt ice. 

The temperature remains at 0 °C until all the liquid water has changed to ice and then begins to drop again as cooling continues. At 1 atm pressure, water freezes (or, in fhe opposite process, ice melts) at 0 °C. This is called the normal freezing point of wafer. Liquid and solid wafer can coexist indefinitely if fhe femperafure is held af 0 °C. However, af femperatures below 0 °C liquid wafer freezes, while af femperafures above 0 °C ice melfs. 

---

Besides the chemical composition, porosity is another property of stone which has great influence on its preservation. An increased porosity increases the exposed surface and pores allow movement of materials such as water and its solutes through the stones. If the pores are blocked or reduced in diameter such substances may be trapped within resulting in increased local interior damage. Exposure to the climatic elements is one important source of decay. Freeze-thaw cycles, in particular, result in pressures on the pore walls of the stone s interior from changes in volume during the phase transition... 

Tank bottom slope is important because sediment, water, and heavy phases settle at the bottom. Corrosion is usually the most severe at the bottom, and the design of the bottom can have a significant effect on the life of the tank. In addition, if the Hquid stock is changed, it is usually desirable to remove as much as the previous stock as possible. Therefore, designs that allow for the removal of water or stock and the ease of tank cleaning have evolved. In addition, specialized tank bottoms have resulted from the need to monitor and detect leaks. Tank bottoms in contact with the soil or foundations are one of the primary sources of leaks from aboveground tanks. 

When running hydrolysis reactions in the liquid phase, it was observed that the sodium could be washed from the bed by flushing with water and monitoring the change in pH of the bed effluent. If this precaution was not observed, initial low yields of product were observed until sodium was entirely removed from the bed. 

Figure 3 shows the dynamic IFT of soybean oil/water interfaces under expansion with constant flow rate as a function of the relative change of the interfacial area, with various surfactants in the oil and aqueous phases, respectively. The IFT is lowest if both phases contain surface active additives, and it hardly changes due to the presence of the fast adsorbing, low molecular emulsifier SPAN 80 in the oil phase. The increase of the dynamic IFT with the interface expansion is most pronounced with 0.01 % BSA in the aqueous phase due to the slow adsorption of the protein. 

Calorimetric forms of the Ng equation were used by Willson et al. (7) to analyse the solid-state degradation of L-ascorbic acid. Known amounts (0.5 g) of dry L-ascorbic acid were placed in ampoules along with known quantities of water and the heat changes in the samples were recorded using an isothermal microcalorimeter. The power-time data obtained were analyzed using the calorimetric form of the Ng equation and the parameters obtained are shown in Table 7. It was shown that, at low added quantities of added water, the reaction could satisfactorily be described by solid-state kinetics, but at higher added quantities of water (more than 500 pL) the reaction was best described by solution phase kinetics. 

Contaminants in the soil compartment are associated with the soil, water, air, and biota phases present. Transport of the contaminant, therefore, can occur within the water and air phases by advection, diffusion, or dispersion, as previously described. In addition to these processes, chemicals dissolved in soil water are transported by wicking and percolation in the unsaturated zone.26 Chemicals can be transported in soil air by a process known as barometric pumping that is caused by sporadic changes in atmospheric pressure and soil-water displacement. Relevant physical properties of the soil matrix that are useful in modeling transport of a chemical include its hydraulic conductivity and tortuosity. The dif-fusivities of the chemicals in air and water are also used for this purpose. 

If the temperature in a citrus orchard drops below -2°C for several hours, the fruit will freeze and be destroyed. Citrus growers spray tiny droplets of heated water to protect the crop if a freeze is predicted. Part of the protection comes from the heat released as the heated water cools. However, much of the heat that protects trees from freezing is released as the water freezes. The phase change from liquid to solid releases 6.01 kilojoules of energy for each mole of water. Creating a layer of ice on the tree actually prevents it from freezing. 

Now that solubility and vapor pressure have been defined, consider how a volatile chemical partitions, or distributes itself, between water and air phases at equilibrium. In general, a partition coefficient is the ratio of the concentrations of a chemical in two different phases, such as water and air, under equilibrium conditions. The Henry s law constant, H (or KH), is a partition coefficient usually defined as the ratio of a chemical s concentration in air to its concentration in water at equilibrium. [Occasionally, a Henry s law constant is interpreted in an inverse fashion, as the ratio of a chemical s concentration in water to its concentration in air see, e.g., Stumm and Morgan (1981, p. 179). Note that in that table, KH is equivalent to 1/H as H is defined above ] Values of Henry s law constants are tabulated in a variety of sources (Lyman et al, 1990 Howard, 1989, 1991 Mackay and Shiu, 1981 Hine and Mookerjee, 1975) Table 1-3 lists constants for some common environmental chemicals. When H is not tabulated directly, it can be estimated by dividing the vapor pressure of a chemical at a particular temperature by its aqueous solubility at that temperature. (Think about the simultaneous equilibrium among phases that would occur for a pure chemical in contact with both aqueous and gas phases.) Henry s law constants generally increase with increased temperature, primarily due to the significant temperature dependency of chemical vapor pressures as previously mentioned, solubility is much less affected by the changes in temperature normally found in the environment. 

Change the elution solvent so that the interferences remain on the solid phase and the analyte is eluted. An example is the sorption of a herbicide from water and its elution from a reversed-phase sorbent, C-18, using ethyl acetate rather than methanol. Ethyl acetate does not remove the majority of the natural organic substances (humic substances)" from the sorbent, while methanol does. Thus, the chromatogram is considerably cleaner with the ethyl-acetate eluent. 

---