DETECTION OF ACIDS. 223 



Soluble carbonates are consequently reagents for soluble zinc 

 salts, while at the same time soluble zinc salts are reagents for 

 soluble carbonates. 



For similar reasons soluble zinc salts are, according to Table 

 XL, reagents for soluble phosphates, arseniates, arsenites, hy- 

 drates, and sulphides, but not for iodides, chlorides, sulphates, 

 nitrates, or chlorates. 



The insolubility of a compound in water is not an absolute 

 guide for preparing this compound according to the general 

 rule given, there being some exceptions. 



For instance : Cupric hydrate is insoluble in water ; by adding 

 solution of cupric sulphate to any soluble hydrate, therefore, the 

 insoluble cupric hydrate should be precipitated, and is precipi- 

 tated by the soluble hydrates of potassium and sodium, but not 

 by the soluble hydrate of ammonium, on account of the forma- 

 tion of the soluble ammonium cupric sulphate. 



There are not many such exceptions, and to mention them in 

 the table would have greatly interfered with its simplicity, for 

 which reason they have been omitted. 



For the same reason some compounds, which are not known 

 at all, have not been specially mentioned. For instance, ac- 

 cording to Table XL, aluminium carbonate and chromium car- 

 bonate are insoluble salts ; actually, however, these compounds 

 do not exist, the affinity between the weak carbonic acid and 

 the feeble bases not being sufficient to unite them. 



It may be finally stated that no well-defined line can be drawn 

 between soluble and insoluble substances. There is scarcely 

 any substance which is not slightly soluble in water, and many 

 of the so-called soluble substances are but very sparingly soluble, 

 as, for instance, the hydrate and sulphate of calcium. 



