380 ANALYTICAL CHEMISTRY. 



higher or lower state of oxidation. A copper bead may thus be changed from 

 blue to red, or red to blue, the blue bead containing the copper in the cupric, 

 the red bead in the cuprous form. In some cases microcosmic salt, NaNH 4 HPO 4 , 

 is used for making the bead. 



8. Liquefaction of solid substances. Most solid substances 

 have to be dissolved for analysis. The solution obtained may be 

 either a simple or chemical solution. In a simple solution the dis- 

 solved body retains all of its original properties, with the exception 

 of its shape, and may be re-obtained by evaporation. Sodium 

 chloride and sugar dissolved in water form simple solutions. A 

 chemical solution is one in which the chemical composition of the sub- 

 stance has been changed during the process of dissolving, as, for 

 instance when calcium carbonate is dissolved in hydrochloric acid ; 

 this solution now contains and leaves on evaporation calcium 

 chloride. The solvents used are water, or the mineral acids for 

 substances insoluble in water, especially dilute, or, if necessary, strong 

 hydrochloric acid. The dissolving action of the acid should be facil- 

 itated by the aid of heat. Nitric or even nitro-hydrochloric acid 

 may have to be used in some cases. 



Three mistakes ore frequently made by beginners in dissolving sub- 

 stances in acids , viz. : The substance is not powdered as finely as it 

 should be ; sufficient time is not given for the acid to act ; too large an 

 excess of the acid is used. 



If a substance is partly dissolved by water and partly by one or 

 more other solvents, it may be well to examine the different solutions 

 separately. 



Substances insoluble in water and in acids have to be rendered 

 soluble by fusion with a mixture of potassium and sodium carbonate, 

 or with potassium acid sulphate, or by the action of hydrofluoric 

 acid. 



The insoluble sulphates of the alkaline earths, when fused with the 

 alkaline carbonates, are con verted into carbonates, while the sulphates 

 of the alkalies are formed. The latter compounds may be eliminated 

 by washing the fused mass with water and filtering : the solid residue 

 upon the filter contains the carbonates of the alkaline earths, which 

 may be dissolved in hydrochloric acid. 



Insoluble silicates may be decomposed by the methods mentioned 

 on page 186. 



