54 IONISATION 



electrostatic forces. A very large amount of energy must there- 

 fore be expended in separating these atoms. Energy is also 

 expended in removing one of the valence electrons from each 

 cation. This energy is supplied by the energy of combination 

 of the electronegative molecules of the solvent (heats of dilution 

 and hydration) and by the heat of combination of the anions with 

 the freed valence electrons. It is thus that Sir J. J. Thomson 

 explains the charges of the ions. The ionising power of a solvent 

 seems to be some function of its dielectric constant. On the other 

 hand, the degree of ionisation of a salt seems to be due to the 

 electronic structure of its constituent atoms. The whole subject 

 is bristling with difficulties and so far explanations can only be 

 regarded as reasoned guesses. 



Water. 



The solutions dealt with above have all been aqueous. Solu- 

 tions in water as the solvent were early recognised as the most 

 important. According to the old Greek philosophers water was 

 " the beginning of all things." Thales said, " All things have 

 their origin in water and return unto the same." The importance 

 of water as a solvent was generally recognised early in the seven- 

 teenth century. Aqueous solutions are fundamental for all 

 biological phenomena. The physical properties of water are in 

 general extreme their numerical expressions are either extremely 

 large or extremely small, and usually the former. Its specific 

 heat and its dielectric constant are the highest of any of the more 

 common liquids. Therefore, water should have a very high 

 ionising power as a solvent. 



One has been accustomed to look upon water as a simple inert 

 substance, of the chemical formula H . OH and with a molecular 

 weight of 18. Physical chemists have proved that this conception 

 does not account for all the properties of water. Lewis and also 

 Langmuir, from thermodynamical principles and also from the 

 study of the colligative properties (see p. 43) of water, have 

 constructed diagrams of the molecule of water. Discussion of 

 this work is somewhat without the bounds of this book. 



In recent years it has been amply demonstrated that a tri- 

 atomic molecule could not possess the properties of water. For 

 instance, 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 tempera- 



