IONISATION CONSTANT 55 



ture of steam should be 100 C. The inference from this is 

 that the molecule of water is bigger than H 2 O. Each simple 

 molecule or hydrol is supposed to combine with another hydrol 

 so as to form a dihydrol or three hydrols may polymerise to 

 trihydrol, and so on. Water, as we know it, consists of a mixture 

 of these various hydrols. The relative amount of each kind is 

 determined (a) by the temperature of the fluid, and (b) by the 

 substances present in solution or, in a less degree, in suspension. 



(a) Temperature controls the kinetic energy of the molecules 

 and so the size of the intra-molecular spaces. Increase of tem- 

 perature, therefore, by increasing the kinetic energy will cause a 

 disruption of polyhydrol into its simpler constituents. Decrease 

 of temperature has the reverse effect. Theoretically, there is 

 the gas H 2 O and the solid (H 2 O) 3 , and between these extremes 

 the liquid (a(H 2 O) +fr(H 2 O) 2 +c(H 2 O) 3 ), a, b and c being 

 constants dependent on the temperature. At each temperature 

 there is equilibrium between the amounts of the various hydrols. 

 The temperature of water has thus an importance in deciding 

 its physical and chemical properties, and therefore, in all reactions 

 involving water, temperature should be stated. 



(b) As has been pointed out above, there is a certain equilibrium 

 composition of water at each temperature. This equilibrium is 

 disturbed by the presence of a solute, especially if it is dissociated. 

 Hydrol is abstracted to hydrate the ions or molecules of the 

 solute and a rearrangement of equilibrium takes place. 



lonisation Constant. 



Absolutely pure water is almost, but not quite a non-electrolyte. 

 As absolutely pure water has not yet been prepared, this is a 

 deduction from the behaviour of water under certain circum- 

 stances. 



Water is ionised according to the equation 



According to Guldberg and Waage's Law of Mass Action (p. 61 ), 

 the product of the concentrations of the reacting substances, 

 H+ and OH~, bears a direct relationship to the mass of the 

 resultant substance H 2 O. 



That is [H+lx[OHl- ^ 



=constant Kw, 



[H 2 0] 



where [H+] =the fraction of the total water which is disso- 



ciated into hydrogen ions. 



