DETECTION OF ACIDS. 243 



tions of the presence or absence of certain acids, that this treatment 

 should never be omitted when a search for acids is made. 



When the substance under examination is liquid, a portion should 

 be evaporated to dryness, and, if a solid residue remains, it should 

 be treated in the same manner as a solid. 



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 either are 

 decomposed, with liberation of the acid (which may escape in the 

 gaseous state), or with liberation of volatile products (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 hydro- 

 chloric acid, the magnesium test for phosphoric acid cannot be used, 

 because this test can be applied to a neutral or an alkaline solution 

 only ; in attempting, however, to neutralize the hydrochloric acid 

 solution, calcium phosphate itself is re-precipitated. 



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

 300 of the most important inorganic salts, oxides, and hydroxides, 

 will greatly aid the student in studying this important feature. It 

 will also guide him in the analysis of inorganic substances, 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 hydroxide) which is insoluble in water may be produced and pre- 

 cipitated by mixing two solutions, one containing the metal, the other 

 containing the acid of the insoluble salt to be formed. For instance : 

 Table XI. states that the carbonates of most metals are insoluble in 

 water. To produce, therefore, the carbonate of any of these metals 

 (zinc, for instance) it becomes necessary to add to any solution of 



