WATER, ITS PROPERTIES AND FUNCTIONS 235 



for the view that the phenomenon is due to the existence of a third compound, 

 steam or monohydrol, is too long for the present work. One or two main facts 

 should be given on account of their importance. 



The "Solution Volume" of a given solute is the increase in volume of the solvent when 

 1 g. of the solute is dissolved in 100 c.c. of the liquid. Thus, when -1 g. of sodium 

 chloride is dissolved in 100 c.c. of water, the volume of the solution is 100'2. c.c., so that 0"2 

 c.c. is the solution volume of 1 g. of sodium chloride. It might be supposed that this 

 would be the volume of the salt in the liquid state, but this cannot be so, since the volume 

 changes with concentration. Moreover, sodium hydroxide has a negative solution volume at 

 certain temperatures and concentrations, so that 140 g. of the solid can be added to a litre of 

 water at without increasing the volume at all, keeping the temperature at 0, of course. 

 It is evident that changes take place in the solvent itself. 



The contraction produced on dissolving is greatest in presence of large excess of the solvent, 

 just as the number of molecules of water in the hydra ted solute is greater the more dilute the 

 solution. The most reasonable explanation of the contraction is, then, that the combined 

 water has a greater density than normal water ; a view indeed supported by other evidence. 



Further, the degree of contraction with the same volume of solvent varies with the 

 temperature, but in such a manner as to show a maximum at a particular temperature, which 

 itself naturally varies with the degree of hydration -of the solute used. In most cases investi- 

 gated by Bousfield and Lowry, the temperature at which this maximum occurs is about 60. On 

 passing from solutes with a small affinity for water to those with a strong one, the maximum 

 is reached at lower and lower temperatures ; in the case of lithium chloride at 35. The 

 deviations thus have their origin at the higher temperatures and extend gradually downwards. 



Now, in the case of the point of maximum density of water at 4, we have seen 

 that the most satisfactory explanation rests on the presence in liquid water of a 

 polymer, identical with ice, which diminishes in concentration as the temperature 

 rises. Similarly, to explain the changes in the volume of the water taken up in 

 hydration of solutes, it is in accord with all facts to assume that, as the temperature 

 rises, there is an increasing formation of a third component of low density, and a 

 partial destruction of this when a hydrate-forming salt is added. It is natural to 

 regard this third component as being identical with steam, that is, monohydrol, and, 

 if this is so, the component intermediate between steam and ice must be dihydrol. 



To sum up, we arrive at the conclusion that liquid water is a system of three 

 components ice, or trihydrol, which is present in greatest concentration at the 

 freezing point; dihydrol, the main component at ordinary temperatures; and 

 monohydrol, or steam, increasing ' as the temperature rises to the boiling point. It 

 is to be remembered that, at any temperature, there will be a certain definite 

 relative proportion of all three of these substances, although at the freezing point 

 monohydrol is probably nearly absent, while trihydrol is nearly absent at the 

 boiling point. 



It is probable, as already remarked, that these three constituents must be 

 looked upon as distinct chemical individuals, although easily converted into one 

 another by small changes of conditions. Thus, regarding the quadrivalence of 

 oxygen as an established fact, trihydrol may be represented : 



1*2 



dihydrol : H 2 = = = H 2 



and in monohydrol, H 2 O, two of the affinities of oxygen must mutually satisfy one 

 another. 



Armstrong (1908) prefers the name "hydrone" instead of "hydrol" to express the simple 

 molecule and dihydrone, etc. , for the polymers. The reason is that water belongs not to the 

 class of alcohols, but rather to that of the ketones. Strictly speaking, this is no doubt correct, 

 but, on the other hand, water may conveniently be regarded as the simplest of the alcohols, 

 if we consider OH as the characteristic group of the class. 



Armstrong assumes further that there is present in water an isomeric form of dihydrone, 

 in which one of the molecules is resolved into H and OH, with increased chemical activity. 

 Thus, dihydrone being 



H 



H H 



hydronol is : 



H M 



