ACIDS, BASES, AND SALTS 173 



Exactly the same type of reaction takes place when we add to 

 the solution of a salt any metal higher up in the activity series 

 than the positive radical of the salt. For example, zinc will 

 displace copper from a solution of cupric sulphate or any other 

 soluble cupric salt: 



Zn + CuSO 4 -> ZnSO 4 + Cu. 



The copper is obtained as a red precipitate. This principle is 

 extensively used in the purification of the more valuable metals 

 at the foot of the activity series (p. 54). Thus copper will displace 

 silver, and silver will displace gold, from solutions of their re- 

 spective salts. 



Similarly, a simple negative radical can be displaced by a more 

 active element. Thus the iodide radical in potassium iodide is 

 displaced by gaseous chlorine, iodine being liberated : 



C1 2 -2KC1 + I 2 . 



2. Double Decomposition. Several examples of this type 

 of reaction between acids, bases and salts in solution have already 

 been discussed (pp. 126, 132, 166 and 168). In fact, whenever 

 two solutions of such substances, which contain no radical in 

 common, are mixed, a double decomposition occurs. Any acid 

 or base will therefore react with any salt of a different acid or base. 

 Any acid will give a reaction with any base. Salts containing 

 no common radical will also react in pairs. 



Very frequently we obtain instant evidence of such reactions 

 by the appearance of a precipitate, one of the products formed 

 being only slightly soluble in water. For example: 



Salt and acid : AgN0 3 + HC1 -^ AgCl J + HNO 3 (1) 



Salt and base: CuCl 2 + 2NaOH - Cu(OH) 2 | + 2NaCl (2) 



Acid and base: H 2 SO 4 + Ca(OH) 2 - CaSO 4 J, + 2H 2 O (3) 



Salt and salt : AgN0 3 + NaCl -4 AgCl J + NaNO 3 (4) 



Such precipitations furnish us with very useful tests for estab- 

 lishing the presence or absence of certain radicals in an unknown 



