THE RECOGNITION OF SUBSTANCES, II, ETC. 537 



A salt which gives an acid reaction must be an acid salt of a 

 poly basic acid (p. 260), or a derivative of a strong acid and a weak 

 base. Similarly a salt which reacts alkaline must be a basic salt, 

 or a derivative of a weak acid and a strong base. 



Recognition by React' ons in Solution. Starting with the 

 substance in solution, its identity can be ascertained by using 

 reactions involving mainly precipitations and oxidations or reduc- 

 tions, which separate the metals into five distinct groups. 



The following plan, taken in conjunction with the statements in 

 the context, shows how a single cation may be identified. What 

 will be said applies only to the case of a solution containing salts 

 like the chlorides, nitrates, or sulphates of one or more cations, 

 and leaves the oxalates, phosphates, cyanides, and some other 

 salts, out of consideration. 



Before attempting to understand this plan, the student should 

 turn to the discussion on ionic equilibria (Chapter XXXVIII) 

 and read it carefully through. The sections on the solubility of 

 precipitates (pp. 460-4) should be particularly studied, since upon 

 the principles therein formulated the whole plan is directly based.* 



Group I. Add to the solution hydrochloric acid. A precipi- 

 tate indicates that cations giving insoluble chlorides are present. 

 Silver, mercurous and lead salts give the white AgCl, HgCl, and 

 PbCl2 respectively. The last-named salt, being appreciably 

 soluble, will be only incompletely precipitated. 



These three chlorides can be distinguished from one another 

 very easily. When excess of ammonium hydroxide is added to 

 the precipitate, silver chloride dissolves (p. 521). Mercurous 

 chloride turns black, owing to the formation of a finely divided 

 mixture of free mercury and mercuric amidochloride Hg(NH 2 )Cl. 

 Lead chloride remains apparently unchanged. 



* Many experimental details, essential for the successful performance of 

 the tests described in this chapter, are here omitted. They will be found 

 in the Authors' Laboratory Outline of Intermediate Chemistry. 



