186 PRINCIPLES OF CHEMISTRY 



every step. We will now consider the reactions of oxides in the state 

 of hydrates, not losing sight of the fact that water is itself an oxide 

 with definite properties, and has. therefore, no little influence on the 

 course of those changes in which it takes part. 



If we take a definite quantity of an acid, and add an infusion of 

 litmus to it, it turns red ; the addition of an alkaline solution does not 

 at once alter the red colour of the litmus, but on adding more and 

 more of the alkaline solution a point is reached when the red colour 

 changes to violet, and then the further addition of a fresh quantity of 

 the alkaline solution changes the colour to blue. This change of the 

 colour of the litmus is a consequence of the formation of a new com- 

 pound. This reaction is termed the saturation or neutralisation of 

 the acid by the base, or vice versa. The solution in which the acid 

 properties of the acid are saturated by the alkaline properties of the 

 base is termed a neutral solution. Such a solution, although derived 

 from the mixture of a base with an acid, does not, however, exhibit 

 either the acid or basic reaction on litmus, yet it preserves many other 

 signs of the acid and alkali. It is observed that in such a definite 

 admixture of an acid with an alkali, besides the change in the colour 

 of litmus, there is a heating effect i.e., an evolution of heat which is 

 alone sufficient to prove that there was chemical action. And, indeed, 

 if the resultant violet solution be evaporated, there separates out, not 

 the acid nor the alkali originally taken, but a substance which has 

 neither acid nor alkaline properties, but is usually solid and crystal- 

 line, having a saline appearance ; this is a salt in the chemical sense of 

 the word. Hence it is derived from the reaction of an acid on 

 an alkali, and through a definite relation between the acid and 

 alkali. The water here taken for solution plays no other part than 

 merely facilitating the progress of the reaction. This is seen from the 

 fact that the anhydrides of the acids are able to combine with basic 

 oxides, and give the same salts as do the acids with the alkalis or 

 hydrates. Hence, a salt is a compound of definite quantities of an 

 acid with an alkali. In the latter reaction, water is separated out if 

 the substance formed be the same as is produced by the combination of 

 anhydrous oxides together. 51 Examples of the formation of salts from 

 acids and bases are easily observed, and are very often applied in 



51 That water really is separated in the reaction of acid 011 alkaline hydrates, ni;iy In- 

 shown by taking some other intermediate hydrate for instance, alumina instead of 

 water. Thus, if a solution of alumina in sulphuric acid be taken, it will have, like the 

 acid, an acid reaction, and will therefore colour litmus red. If, on the other hand, a 

 solution of alumina in an alkali for instance, potash be taken, it will have an alkaline 

 reaction, and will turn red litmus blue. On adding the alkaline to the acid solution 

 until neither an alkaline nor an acid reaction is produced, a salt is formed, consisting of 



