640 SMITH'S INTERMEDIATE CHEMISTRY 



tests which may serve to confirm the conclusions arrived at in 

 the course of the above analysis. Often the metallic radical can 

 be recognized by a displacement reaction (compare pp. 514, 517). 

 Often, again, color changes which occur during the course of the 

 scheme of operations outlined above give indications of value. 

 Thus a yellow solution changing to green during the passage of 

 H 2 S in Group II signifies a chromate (p. 533), a purple solution 

 becoming colorless signifies a permanganate (p. 529). 



Tests for Negative Radicals. Precipitation reactions sim- 

 ilar in nature to those outlined above may be utilized as additional 

 tests for the negative radical of an unknown substance (see chap- 

 ter XXXII). Thus a chloride gives with silver nitrate a white 

 precipitate, soluble in ammonia. A sulphate gives a white pre- 

 cipitate with barium chloride, insoluble in hydrochloric acid. 

 From the context, the student will be able to discover for himself 

 confirmatory tests of this kind for most of the common negative 

 radicals discussed in this book. For a complete scheme, how- 

 ever, a manual of qualitative analysis should be consulted. 



Insoluble Substances. If the unknown substance is insol- 

 uble in water, try to bring it into solution by boiling successively 

 with dilute HN0 3 , concentrated HN0 3 , and aqua regia (p. 310). 

 In case it dissolves, evaporate off the excess of acid and proceed 

 with the analysis as above. The only common substances which 

 are still insoluble are: the sulphates of Pb, Sr, Ba; certain mineral 

 oxides such as A1 2 3 , Cr 2 O 3 , Fe 2 O 3 , Sn0 2 ; some silicates; CaF 2 ; 

 AgCl (soluble in NH 4 OH). Fuse with NasCOs in a crucible, cool, 

 extract with water, and filter. 



The residue contains the positive radical as carbonate, and may 

 be analyzed for this after dissolving in HN0 3 . The filtrate con- 

 tains the negative radical as sodium salt, and may be examined 

 as in Chapter XXXII. 



