DETECTION OF ACIDS. 221 



Most non-volatile, organic substances (including most organic 

 acids) color sulphuric acid dark when heated with it. 



Dry inorganic salts when heated with sulphuric acid are 

 either decomposed, with liberation of the acid (which may 

 escape in the gaseous state), or with liberation of volatile pro- 

 ducts (produced by the decomposition of the acid itself), or no 

 apparent action takes place. See Table IX. 



Detection of acids by means of reagents added to their neutral 

 or acid solution. Whenever a substance is soluble in water, 

 there is little difficulty of finding the acid by means of Table X.; 

 but if the substance is insoluble in water, and has to be rendered 

 soluble by the action of acids, this table may, in some cases, be 

 of no use, because the acid originally present in the substance 

 may have been liberated, and escaped in a gaseous state (as, 

 for instance, when dissolving insoluble carbonates in acids), or 

 the tests mentioned in the table may refer to neutral solutions, 

 while it is impossible to render the solution neutral without re- 

 precipitating the dissolved acid. If calcium phosphate, for 

 instance, be dissolved by hydrochloric acid, the magnesium test 

 for phosphoric acid cannot be used, because this test can only be 

 applied to a neutral or alkaline solution ; in attempting, how- 

 ever, to neutralize the hydrochloric acid solution, calcium 

 phosphate itself is reprecipitated. 



Table XI., showing the solubility or insolubility (in water) of 

 over 300 of the most important inorganic salts, oxides and 

 hydrates, will greatly aid the student in studying this important 

 feature. It will also guide him in the analysis of inorganic sub- 

 stances, as it gives directions for over 300 (positive or negative) 

 tests for metals, and an equal number for acids. 



To understand this, it must be remembered that any salt (or 

 oxide or hydrate) which is insoluble in water may be produced 

 and precipitated by mixing two neutral solutions, one contain- 

 ing the metal, the other containing the acid of the insoluble salt 

 to be formed. For instance : Table XI. states that the car- 

 bonates of most metals are insoluble in water. To produce, 

 therefore, the carbonate of any of these metals (zinc, for in- 

 stance) it becomes necessary to add to any solution of zinc (sul- 

 phate, chloride, or nitrate of zinc) any soluble carbonate (sodium 

 or potassium carbonate), when the insoluble zinc carbonate is 

 produced. 



