CHEMICAL EQUATIONS, TYPES OF CHEMICAL CHANGE, ETC. 121 



of hydrochloric and nitric acids by the aid of concentrated sulphuric 

 acid is an example of the method, and large quantities of sodium 

 sulphate are obtained as a by-product for the market. 



4. By the action of one salt upon another salt. This method is 

 chiefly used when one of the products is insoluble or very nearly so, 

 and is known as precipitation. Usually the insoluble product is the 

 desired one, but the soluble one may also be isolated. A great many 

 of the analytical reactions, called tests, fall under this method. 

 Nearly all carbonates and phosphates are obtained by precipitation. 



CaCl 2 + Na 2 CO 3 = CaC0 3 + 2NaCl. 

 CaCl 2 + Na 2 HPO, = CaHP0 4 + 2NaCl. 



Calcium carbonate and phosphate are precipitated and may be re- 

 moved. 



An example of the use of the method to get the soluble product is 

 shown by the equation : 



CuS0 4 + BaCl 2 = BaS0 4 + CuCl,. 



A solution of copper chloride is obtained by filtering off the barium 

 sulphate. 



In the case of certain salts it is simpler or more economic to 

 follow special methods, which may be seen under the respective salts. 

 Some of these salts are ferrous iodide, ammonium iodide, sodium 

 hypochlorite, iodide, and thiosulphate, potassium permanganate, 

 dichromate and chlorate, sodium carbonate, mercurous and mercuric 

 chloride, etc. 



According to the number of hydrogen atoms replaced in an acid, 

 we distinguish normal and acid salts. A normal salt is one formed by 

 the replacement of all the replaceable hydrogen atoms of an acid. 

 For instance : Potassium chloride, KC1, potassium sulphate, K 2 SO 4 , 

 potassium phosphate, K 3 PO 4 . (As monobasic acids have but one atom 

 of hydrogen which can be replaced, they form normal salts only.) 



Normal salts often have a neutral reaction to litmus, but they may 

 have an acid or even an alkaline reaction. 



It is found that soluble normal salts derived from a weakly ioniz- 

 ing acid, as carbonic, boric, phosphoric, sulphurous, hypochlorous, 

 silicic, hydrogen sulphide, and a strongly ionizing base, as sodium 

 and potassium hydroxide, and some others, have an alkaline reaction, 

 while those derived from a strongly ionizing acid and a weakly ioniz- 

 ing base, as the hydroxide of many of the heavy metals, such as 

 Fe(OH) 2 , A1(OH) 3 , Cu(OH) 2 , etc., have an acid reaction. The reason 



