Peculiar Properties of Water 97 



negative than in ammonia and that here the hydrogens are even more 

 positive. The water molecule has, roughly, twice the polarity of am- 

 monia. With a molecular weight of 18 it boils at 100° C. The boiling 

 point of ammonia is about half way between those of methane and 

 water. The other physical properties of ammonia also lie intermediate 

 between those of water and methane. 



Association of Water. — The considerable degree of polarity of 

 water is very intimately connected with its peculiarities. When one 

 water vapor molecule (figure 5) comes in contact with another the 

 positive part of the one attracts the negative part of the other and 

 they tend to stick together (figure 6) in a very compact structure. One 

 of the two corners of the aggregate is positive and the other is nega- 

 tive so that this dihydrol, the name that has been suggested for liquid 

 water, is still polar. DihydroP is often spoken of as liquid water. But 

 liquid water is not dihydrol. It is a solution, especially at higher 

 temperatures, of varying amounts of monohydrol, as steam is often 

 designated, in dihydrol. As the temperature falls, there is an ever 

 increasing amount of ice or ice-like molecules dissolved. Trihydrol 

 is the name that has been given to these aggregates. But as we shall 

 see presently the name, trihydrol, should refer to an arrangement of 

 bondings between molecules rather than to the total number of mole- 

 cules in the aggregates. Analogous to the dynamic equilibrium of 

 evaporating and condensing molecules above the surface of water so 

 there is within the liquid a dynamic equilibrium between monohydrol, 

 dihydrol and trihydrol; e. g., 



6H20==3(HoO)2^^2(HoO)3 



The reactions from left to right are exothermic and are favored by 

 lowering of temperature. When the temperature rises the molecules 

 become so active in their movement that the aggregates are broken up. 



The Arrangement of the Atoms in Ice. — It has recently been shown 

 by Bragg^ that in the ordinary ice crystal arrangement of atoms 

 each oxygen is surrounded by four hydrogens at the corners of tetra- 

 hedra and that each hydrogen is situated between two oxygens, joining 

 them together (figure 7). In this manner all the stray fields of force 

 of water are satisfied and the change from dihydrol to ice should be 

 exothermic; the latent heat of freezing of the water is 80 calories per 

 gram. The ice lattice is not as compact as dihydrol so that there is an 

 expansion when this re-arrangement occurs. Similarly the increase 

 in volume below 4° is caused by an ever increasing shift of dihydrol 

 into the larger ice molecules or state of aggregation. 



The specific heat curve for water sheds some light on the equilibrium 

 between dihydrol and trihydrol with respect to the temperature. The 

 extra heat required to shift the equilibrium gives water a high specific 

 heat at 0° and with ever decreasing ice dissolved in the water the 

 specific heat decreases to a minimum at about 22°. Beyond this point 

 the curve rises again as is the general rule for all substances. But here 



3 Symposium on Water. Trans. Faraday Soc. ti, 71-123 (1910). The proposal is 

 made there that the water molecules are coupled together directly through the oxygens. 

 ••Bragg. Proc. Phys. Soc. (London), 31t, 98 (1922). 



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