AMMONIA 305 



ionized), turns red litmus blue, possesses the characteristic soapy 

 taste and feeling, and enters into double decomposition with acids, 

 neutralizing them: 



NH 4 OH + HC1 - H 2 + NH 4 C1. 



The salts, obtained by evaporation, are, of course, identical with 

 those formed by union of ammonia with the same acids. 



Ammonium hydroxide used to be known as " volatile alkali," in 

 reference to the fact that it decomposes into its constituents 

 (NH 3 + H 2 0), both of which are volatile, while the other alkalies 

 (NaOH, etc.) are not volatile (" fixed "). This property was 

 utilized in the laboratory method of making ammonia (p. 302). 



The Salts of Ammonium. The salts are ionized in 

 aqueous solution, giving NH 4 as the positive ion: 



(NH 4 ) 2 S0 4 -* 2NH 4 + + S0 4 =. 



\yhpn fyatedi ^ry, in a. tiihf> T they are decomposed. Most of them 

 give ammonia and an acid: 



NH 4 Cl<=NH 3 T+HClt. 



F t^ Q fl"H ^ o1 sn vnlati 10 at, or below, a red heat, like sulphuric 

 acid H 2 SO 4 , the whole salt usually vavorizes. These actions are 

 reversible (read the equation backwards). HPTI^ ffre acid and 



and the salt condenses a^ain in a cold 





part of the tube.. This behavior helps us to recognize a salt 

 of ammonium, for the salts of mercury are the only others which 

 behave in this way. 



Uses of Ammonia. Some of the uses have already been men- 

 tioned. The ammonia process for making carbonate of soda 

 is described under the latter substance (p. 366). 



Refrigeration by liquid ammonia depends upon the fact that 

 liquid ammonia, like any other liquid, absorbs heat in evapo- 

 rating. It absorbs 260 cal. per gram. To freeze one gram of 

 water at 0, 80 calories have to be subtracted. Thus, 1 gram 

 of liquid ammonia in evaporating will convert about 3 grams of 



