DETECTION OF ACIDS. 393 



zinc (sulphate, chloride, or nitrate of zinc) any soluble carbonate 

 (sodium or potassium carbonate), when the insoluble zinc carbonate 

 is produced. 



Soluble carbonates consequently are 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 XI., 

 reagents for soluble phosphates, arsenates, arsenites, hydroxides, 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 

 above for the precipitation of insoluble compounds, there being some 

 exceptions. 



For instance : Cupric hydroxide is insoluble in water ; therefore, 

 by adding solution of cupric sulphate to any soluble hydroxide, the 

 insoluble cupric hydroxide should be precipitated, and is precipitated 

 by the soluble hydroxides of potassium and sodium, but not perma- 

 nently by the soluble hydroxide of ammonium, on account of the 

 formation 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, according to Table 

 XI., aluminum carbonate and chromium carbonate are insoluble salts : 

 actually, however, these compounds can scarcely be formed, the 

 affinity between the weak carbonic acid and the feeble bases not 

 being sufficient to unite them. Also, bismuth nitrate and a lew 

 other salts are reported as soluble, while actually they suffer a 

 decomposition by water. 



Finally, it may be 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 hydroxide and sulphate of calcium. 



Table XII. shows the solubility of a large number of compounds 

 more accurately than Table XL ; it may be used for reference. 



