WATER, ITS PROPERTIES AND FUNCTIONS 229 



AS A SOLVENT 



When it is said that chemistry has been built up almost entirely on aqueous 

 solutions, it is not to be understood that water has been used as a solvent 

 merely because of its cheapness and accessibility, but that it has unique properties 

 in this respect. In fact, there is no other liquid capable of dissolving so great 

 a variety of substances. As regards inorganic salts, very few are soluble in any 

 other liquid. Of organic substances, more are to be found which require alcohol, 

 ether, and so on for solution, but, even here, the majority can be dissolved in 

 water. 



Geological facts are, perhaps, the most striking evidence of the efficiency of water as 

 a solvent, but details are out of place here. It is sufficient to recall the fact (L. J. Henderson, 

 1913, p. 113) that the total amount of dissolved matter carried by the rivers of the world 

 to the sea amounts to five thousand million tons per annum. 



Turning to the living organism itself, a list of the substances found in urine, 

 which were practically all previously in solution in the blood, illustrates the 

 variety of chemical compounds soluble in water. These are : urea, carbamic 

 acid, creatinine, creatine, uric acid, xanthine, guanine, hypoxanthine, adenine, 

 oxalic acid, allantoin, hippuric acid, phenaceturic acid, benzorc acid, phenolsulphuric 

 acid, indoxylsulphuric acid, paraoxyphenylacetic acid, urobilin, urochrome, uroery- 

 thrin. hsematoporphyrin, glucose, lactose (when the mammary glands are active), 

 glycuronic acid, glycine, alanine, leucine, tyrosine, various enzymes, putrescine, 

 cadaverine, chlorides, bromides, iodides, phosphates, sulphates, salts of potassium, 

 sodium, ammonia, calcium, magnesium, iron, carbonic acid, nitrogen, argon and 

 other substances. In pathological conditions : proteins, oxybutyric and acetoacetic 

 acids, acetone and, in some abnormalities of metabolism, cystine and homo- 

 gentisic acid. Only a few of these are soluble in other liquids to any extent, 

 even in alcohol. 



Chemical Stability. With the exception of hydrolytic and electrolytic dissocia- 

 tion, the action of water upon solutes is practically nil. This depends upon its 

 chemical inertness and stability. Substances can therefore be recovered, by 

 evaporation of the solvent, in their original state. This applies also to substances 

 which undergo electrolytic dissociation, since the ions reunite on concentration ; 

 and even to some extent to hydrolytically dissociated solutes, when the products 

 are non- volatile. 



Solubility. As to what happens in the actual process of solution, we are, as 

 yet, very much in the dark. Why, for example, sodium salts are nearly all 

 soluble in water, whereas certain corresponding potassium salts are insoluble, and 

 why the nitrates of practically all metals are freely soluble, but only the chlorides 

 of some of them, is not explained. The fact itself is of great importance in the 

 production of osmotic pressure. When dissolved, the molecules of a substance 

 are free to manifest the effects of the energy due to their movement. The process 

 is, in fact, a " dispersion " of the same kind as that more or less visible and obvious 

 in the case of colloidal solutions, differing only in the degree of subdivision. 



The history of the various theories proposed is of much interest and may be read in the 

 account given by Walden (1910). We note that there has been much argument between the 

 adherents of physical and of chemical theories. The question has, from the first, been closely 

 connected with that of the nature of chemical affinity, so that as molecular physics made 

 further and further strides, attempts were made repeatedly at physical explanations of 

 chemical affinity. In the case of solution, as we shall see presently, there is undoubted 

 evidence of combination of some kind between solvent and solute, " hydration " or 

 " solvation " ; but, since the chemical properties of a substance suffer little or no change 

 in the process, it seems rather a matter of words whether we choose to consider the process as 

 one of satisfaction of "residual affinities" or prefer to speak of attractive forces between 

 molecules ; neither, in fact, goes far towards an explanation and the different modes of 

 expression serve equally well at present and will probably appeal differently to investigators 

 according to whether they are chiefly occupied with the physical or chemical aspects of the 

 phenomena. In the distant future they will, no doubt, be reconciled ; although, presumably, 

 it must be admitted that the explanation will most likely be in a better knowledge of 

 the physics of the atom. 



As dilute solutions are of frequent use in physiological work, and changes in 

 their concentration require to be known, it may be useful to refer to the delicate 



