Peculiar Properties of Water 99 



ions. But the study of crystal structures by X-rays shows that the 

 sodium and chlorine atoms in the crystal lattice are already polarized. 

 The mechanism of solution involves the attraction of the polar water 

 to the surface of the salt molecules. The positive part being- attracted 

 to the chloride ions and the negative part to the sodium ions. Other 

 water molecules shove in between the ions and separate them. This 

 separation, of course, requires energy which is made up in part from 

 the heat of the hydration of the ions and in part from the surroundings 

 as is evidenced by the cooling of the water as it dissolves the salt. 

 With some substances, on dissolving, the heat of hydration exceeds that 

 required to separate the ions. The strong acids and bases and several 

 salts like anhydrous sodium and calcium sulfates all go into solution 

 exothermically. Each ion is surrounded with an envelope of hydrated 

 water which aids in holding them apart. This, it seems, is the real 

 cause of the power of water as an ionizing substance rather than its 

 high dielectric constant (specific inductive capacity). 



The dielectric constimt according to the conception of Faraday and 

 Maxwell" is due to the presence of dipoles in the medium which are 

 orientated in an electric field. Other substances of high dielectric con- 

 stant such as formic acid or hydrocyanic acid should, according to 

 the older idea, cause marked ionization of dissolved water; but experi- 

 ments of P. Walden" proved that this is not the case. If the dielectric 

 constant of a material depends upon Faraday's dipoles and if secondary 

 valence combination depends upon polarity there should be a very close 

 parallelism. It is one of the peculiarities of water that there exists a 

 discrepancy in this respect. It seems to indicate that water possesses 

 an exceptionally marked tendency to enter into those hydration reac- 

 tions which are really physico-chemical; and this is just the point! 

 This type of reactions is controlled, not merely by the attraction of 

 unlike poles in different molecules, but also by the specific chemical 

 character of the molecules themselves. Now the chemical character of 

 the water molecule is such that it seems to bring about the greatest 

 number of hydration reactions. This is to be connected with the small 

 size of the water molecule in which the hydrogens occupy but a small 

 fraction, as well as to the two pairs of extra electrons of the oxygen. 

 This specificity of character is, I believe, confirmed by the remarkable 

 effect on colloidal systems of the ions formed from water, namely, the 

 hydrogen and hydroxyl ions. Jacques Loeb' has emphasized the im- 

 portance of their relative concentrations (pH) in all colloidal work. 



According to the ideas here expressed solubility is sometimes a 

 matter of secondary valence combination." It is for water, I feel sure, 

 but there are many other solutions which do not exhibit any evidence 

 of secondary valence combination. If two vessels containing diflTerent 



5 Jeans, Electricity and Magnetism, 1th ed., pp. 126-135 (1920). 



« Cit. 3, p. 71-123. 



' Loeb. Proteins and Theory of Colloidal Behavior (1922). Science, 56", 731 (1922). 



* Secondary valence combination is due to the extra electrons in the oxygen, nitrogen 

 or other atoms. Hydration is a typical example. When a molecule of dissolved sub- 

 stance is hydrated in water solution a rather indefinite envelope of water surrounds 

 the molecule. In solid hydrated crystals a stochiometrical amount of water is in sec- 

 ondary valence combination. 



