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3 Liquid state The molecules in liquid water also are held together by hydrogen bonds. When ice melts, many of the hydrogen bonds are broken, and those that remain are not numerous enough to keep the molecules in a regular arrangement. Many of the un- usual properties of liquid water may be understood in terms of the hydrogen bonds which remain. As water is heated from 0°C, it contracts until 4°C is reached and then begins the expansion which is normally associated with increasing temperature. This phenomenon and the increase in density when ice melts both result from a breaking down of the open, hydrogen-bonded structure as the temperature is raised. The viscosity of water decreases tenfold as the temperature is raised from 0 to 100°C, and this also is associated with the decrease of icelike character in the water as the hydrogen bonds are disrupted by increasing thermal agitation. Even at 100°C, the hydrogen bonds influence the properties of water strongly, for it has a high boiling point and a high heat of vaporization compared with other substanc- es of similar molecular weight. The electrical conductivity of water is at least 1,000,000 times larger than that of most other nonmetallic liquids at room tem- perature. The current in this case is carried by ions produced by the dissociation of water according to reaction (4). H2O H + + OH- This reaction is reversible, and equilibrium is reached rapidly, so there is a definite concentration of H+ and OH− ions in pure water. At 25°C, this concentration is 10-7 mole/liter of each species or about 1014 ions/ml. This concentration of ions is affected by the temperature or by the presence of solutes in the water. Pure water, either solid or liquid, is blue if viewed through a thickness of more than 2 m. The other colors often observed are due to impurities. Solutions in water Water is an excellent solvent for many substances, but particularly for those which dissociate to form ions. Its principal scientific and industrial use as a solvent is to furnish a medium for purifying such substances and for carrying out reactions between them. Among the substances which dissolve in water with little or no ionization and which are very soluble are ethanol and am- monia. These are examples of molecules which are able to form hydrogen bonds with water molecules, although, except for the hydrogen of the OH group in ethanol, it is the hydrogen of the water that makes the hydrogen bond. On the other hand, substances which cannot interact strongly with water, either by ionization or by hydrogen bonding, are only sparingly soluble in it. Examples of such substances are benzene, mercury, and phosphorus. For discussions of another important class of solu- tions in water. Chemical properties Water is not a strong oxidizing agent, although it may en- hance the oxidizing action of other oxidizing agents, notably oxygen. Examples of the oxidizing action of water itself are its reactions with the alkali and alkaline-earth metals, even in the cold; for instance reaction (5), Ca + 2H2O —> Ca2 + + 2OH- + H2 and its reactions with iron and carbon at elevated temperatures, reactions (6) 3Fe + 4H2O —> Fe3O4 + 4H2 and (7). Reaction (7) is used commercially to produce a gaseous fuel from solid coke. C + H2O —> CO + H2 The gaseous mixture, CO + H2, called water gas, is formed when steam is passed over coke heated to 600°C. Water is an even poorer reducing agent than oxidizing agent. One of the few substances that it reduces rapidly is fluorine, but this reaction is complicated. Chlorine is reduced only very slowly in the cold, according to reaction (8). 2Cl2 + 2H2O —> + O2 + 4H + + 4Cl- An example of another sort of oxidation-reduction reac- tion in which water plays an essential role beyond that of the solvent is the disproportionation of chlorine, reaction (9), Cl2 + H2O —> HOCl + H + + Cl- which is fast and incomplete in neutral solution but goes to completion if base is added. Substances with strong acidic or basic character react with water. For example, calcium oxide, a basic oxide, reacts in a process called the slaking of lime, reaction (10). CaO + H2O —> Ca(OH)2 Sulfur trioxide, an acidic oxide, also reacts, reaction (11). SO3 + H2O —> H2SO4 This reaction occurs in the contact process for the manufac- ture of sulfuric acid. Both of these reactions evolve enough heat to produce fires or explosions unless precautions are taken. Water (continued) + ward ' s science + ward ' s science