Chapter I — 5 — Introduction 



As water is warmed, however, thermal expansion increases the volume. Rontgen ex- 

 plained the maximum density as a combination of these two effects. 



The compressibility of a normal liquid decreases with temperature. Rontgen from 

 the association theory suggested that pressure would decrease the number of ice mole- 

 cules ; hence there should be a point of minimum compressibility of water. Such a 

 point occurs at about 50° C. He explained the increase of the thermal coefficient of 

 expansion at pressures around 3,000 atmospheres by the effect of pressure in breaking 

 up ice molecules. He suggested that the point of maximum density would occur at 

 lower temperatures under pressure and that the freezing point would also be lowered. 

 This has proved true. The anomalous decrease in viscosity with increasing pressure 

 he explained on the assumption that the simpler water molecules had a lower viscosity 

 than ice molecules. 



The Hydrols: — In 1900 Sutherland proposed that water vapor is H2O (hydrol), 

 ice pure (H20)3 (trihydrol), and liquid water a mixture of (H20)2 (dihydrol) and 

 (H20)3 in proportions dependent upon the temperature. Figure 1 shows the latter 

 molecules as pictured by Sutherland. At 0° C. the fraction of (H20)3 in water was 

 calculated to be 0.375 ; at 20° C, 0.321 ; at 40° C, 0.284 ; at 60° C, 0.255 ; at 80° C, 

 0.234; and at 100° C, 0.203. At the critical temperature water was supposed to be 

 composed of nearly pure hydrol. At 4° C. Sutherland supposed it to be approxi- 

 mately Vs (H20)3 and Yi (H20)2. 



Sutherland concluded that the latent heat of fusion of ice is mostly a latent heat 

 of dissociation of trihydrol into dihydrol, partly masked by heat of solution of trihydrol 

 in dihydrol ; and that the latent heat of vaporization also includes the heat of dissocia- 

 tion of the dihydrol and trihydrol of water into the hydrol of steam. The specific heat 

 of water is not an ordinary specific heat but includes heat of dissociation. Pressure 

 dissociates trihydrol ; at temperatures between 0° and 100° C. trihydrol is completely 

 dissociated at pressures about 2300 atmospheres. Since pressure causes dissociation, 

 tension should bring about association ; at temperatures below 40° the surface layer 

 of water he thought to be largely trihydrol. 



Sutherland explains the polymerization of hydrol on a tetrad oxygen valence, the 

 assumption being that three H2O molecules of trihydrol are bound into a triangular 

 grouping by the extra oxygen bonds. The sharp melting point of ice depends upon 

 molecular resonance resulting in the breaking of these bonds. 



Armstrong, et al. (1908) proposed the possible existence of water isomers of differ- 

 ent structure, the hydrones. Active water molecules, hydrone (HOH) or hydronol 



/H 

 (H20(' /-^■^T ), take part in chemical reactions whereas inactive molecules formed by 



association are closed and hence chemically inert. Such molecules he diagrammed 

 thus: 



Dihydrone Trihydrone Tetrahydrone 



H2O = OH2 H2O — OH2 H2O — OH2 



\ / II 



O H2O — OH2 

 H2 



Dissociation takes place constantly and is conditioned by temperature and the pres- 

 ence of solutes. (H20)n^n H2O. 



Solution of HCl would produce the following molecules : 



H20/S HCL H2O: CIH 



\'-' \OH 



The first two he considered active, the third inert so long as it remained closed. Dilu- 

 tion should bring about increase in the active forms. 



The Faraday Symposium : — The constitution of water engaged the attention of the 

 Faraday Society in their symposium of 1910. The association theory predominated and 

 most papers concerned estimation or measurement of the degree of association under 

 different conditions. Guye (1910) calculated the association factor assuming that 

 association occurs in both the vapor and liquid states. His values were 80° C, 1.90; 

 100° C, 1.86; 120° C, 1.82. These compared well with values derived from surface 

 tension measurements. Much of the interest in association grew out of work on molec- 

 ular weight determination by physical measurements such as surface tension, cohesion, 

 etc. Guye, however, emphasized the chemical nature of association and used the term 

 polymerization more often than association. 



