CHLORINE. 233 



and has been found in some volcanic gases. One volume of hydrogen 

 combines with one volume of chlorine to form two volumes of hydro- 

 chloric acid. 



For all practical purposes the acid is obtained by the decomposition 

 of a chloride by sulphuric acid : 



NaCl + H 2 S0 4 = HC1 + NaHSO 4 ; 

 or 



2NaCl + H 2 S0 4 = 2HC1 



Experiment 18. Use apparatus as in Fig. 39, p. 168. Place about 20 grammes 

 of sodium chloride into the flask (which should be provided with a funnel-tube) 

 and add about 30 c.c. of concentrated sulphuric acid ; mix well, apply heat, and 

 pass the gas into water for absorption. If a pure acid be desired, the gas has 

 to be passed through water contained in a wash-bottle ; apparatus shown in 

 Fig. 42, page 207, may then be used. Use the acid made for tests mentioned 

 below. How much of the U. S. P. 31.9 per cent, hydrochloric acid can be 

 made from 117 pounds of sodium chloride? 



The liberation of hydrochloric acid from a chloride by sulphuric acid is an 

 example of reversible reactions that run to completion because of the removal 

 of one of the factors that is necessary to maintain an equilibrium (see page 

 114). The character of the reaction is like that in the case of the liberation 

 of nitric acid, the ionic features of which are discussed in Chapter 15. The 

 ionic reaction is this : 



NaCl ^ Na* + C1M ^ HC1 HC1. 

 H 2 SO 4 ^ HSO/ + H- / dissolved gas. 



Hydrochloric acid is a colorless gas, has a sharp, penetrating odor, 

 and is very irritating when inhaled. It is neither combustible nor 

 a supporter of combustion, and has great affinity for water, which 

 property is the cause of the formation of white clouds whenever the gas 

 comes in contact with the vapors of water, or with moist air ; the white 

 clouds being formed of minute particles of liquid hydrochloric acid. 



While hydrochloric acid is a gas, this name is used also for its 

 solution in water, one volume of which at ordinary temperature takes 

 up over 400 volumes of the gas. 



The hydrochloric acid of the U. S. P. is an acid containing 31.9 

 per cent, of HC1. It is a colorless, fuming liquid, having the odor 

 of the gas, strong acid properties, and a specific gravity of 1.158. 

 The official diluted hydrochloric acid is made by mixing 100 parts 

 by weight of the above acid with 219 parts of water. It contains 10 

 per cent, of HCL 



The same antidotes may be used as for nitric acid. 



A 20.2 per cent, solution of hydrochloric acid distils unchanged at 110 C. 

 (230 F.) under 760 mm. pressure. When a more concentrated solution is 

 heated,, it first loses mainly the gas, and a more dilute solution mainly water 



