o2 lONISATION 



it is composed of gases with extremely low freezing and boiling 

 points. Oxygen boils at — 181°, while the figure for hydrogen 

 is — 253° C. From comparison with compounds of known 

 composition, ice should form at — 150° and the temperature of 

 steam should be — 100° C. The inference from this is that the 

 molecule of water is bigger than HgO. 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 tempera- 

 ture, 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 HgO and the solid (H20)3, and between these extremes 

 the liquid (ft^HgO) + ^(H20)2 + c(H20)3), a, b and c being con- 

 stants 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 equilibriiun 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 



HoO — H+ + 0H-. 



According to Guldberg and Waage's Law of Mass Action, 

 the product of the concentrations of the reacting substances, 

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

 resultant substance H2O. 



[H+J X [OH]" 

 That is FTTTTi = constant K, 



