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2 Gaseous state Water vapor consists of water molecules that move nearly independently of each other. The relative positions of the atoms in a water molecule are shown in Fig. 2. The dotted circles show the effective sizes of the isolated atoms. The atoms are held together in the molecule by chemical bonds which are very polar, the hydrogen end of each bond being electri- cally positive relative to the oxygen. When two molecules near each other are suitably oriented, the positive hydrogen of one molecule attracts the negative oxygen of the other, and while in this orientation, the repulsion of the like charges is com- paratively small. The net attraction is strong enough to hold the molecules together in many circumstances and is called a hydrogen bond. When heated above 1200°C, water vapor disso- ciates into H, H2, O, O2, and OH; for example, reaction (3). H2O —> H + OH These products recom- bine completely to form water when the tem- perature is lowered. Water vapor also undergoes most of the chemical reactions of liquid water and, at very high concentrations, even shows some of the unusual solvent properties of liquid water. Above 374°C, water vapor may be compressed to any density without liquefying, and at a density as high as 0.4 g/cm3, it can dissolve appreciable quantities of salt. These conditions of high temperature and pressure are found in efficient steam power plants. Solid state Ordinary ice consists of water molecules joined together by hydrogen bonds in a regular arrangement, as shown in Fig. 3. The circles represent only the positions of the atoms, but if the sizes, as indicated in Fig. 2, are superimposed upon the figure, then it appears that there is considerable empty space between the molecules. This unusual feature is a result of the strong and directional hydrogen bonds taking precedence over all other intermolecular forces in determining the structure of the crystal. If the water molecules were rearranged to reduce the amount of empty space, their relative orientations would no longer be so well suited for hydrogen bonds. This rearrange- ment can be produced by compressing ice to pressures in excess of 14 megapascals. Altogether five different crystalline forms of solid water have been produced in this way, with the form obtained depending upon the final pressure and temper- ature. They are all denser than water, and all revert to ordinary ice when the pressure is reduced. Water (continued) + ward ' s science Property Value Freezing point 0°C Density of ice, 0°C 0.92 g/cm3 Density of water, 0°C 1.00 g/cm3 Heat of fusion 335 J/g Boiling point 100°C Heat of vaporization 2260 J/g Critical temperature 347°C Critical pressure 22.0 MPa Specific electrical conductivity at 25°C 1 × 10-7/ohm-cm Dielectric constant, 25°C 78 Table — Properties of water Fig. 2: Water molecule. Fig. 3: Structure of ice. The hydrogen atoms are omitted for all but two water molecules.