HYDROCHLORIC ACID. CALCULATIONS 131 



When hydrogen chloride is mixed with ammonia NH 3 the gases 

 unite to form a cloud of fine, solid particles of ammonium chloride. 



HC1 + NH 3 -> NH 4 C1 1 . 



Chemical Properties of Hydrochloric Acid. The solution 

 of hydrogen chloride in water is an entirely different substance in 

 its behavior from hydrogen chloride. (1) The solution is sour 

 in taste. (2) It changes the color of litmus, a vegetable coloring 

 matter, from blue to red. (3) It is a conductor of electricity, and 

 is decomposed by the current, hydrogen being liberated at the 

 negative wire (p. 55). (4) When the metals preceding hydrogen 

 in the order of activity (p. 54) are introduced into hydrochloric 

 acid, hydrogen is displaced and liberated. 



In a later chapter (ch. xv) we shall see that these four properties 

 of hydrogen chloride in aqueous solution are properties common to 

 all substances called acids. We may sum up the main proper- 

 ties of a solution of hydrogen chloride in water in one word, there- 

 fore, by saying that it is an acid. 



Hydrochloric acid interacts with many other compounds in 

 solution. In some instances, one of the new substances produced 

 can be seen, because it appears as a precipitate. One such ex- 

 ample has already been discussed in detail (see equation 2, p. 127). 

 When hydrochloric acid is added to a solution of silver nitrate 

 (AgNO 3 lunar caustic), a precipitate of silver chloride (AgCl) 

 is obtained, which is white and curdy in appearance. The other 

 product, nitric acid (HNO 3 ), remains dissolved and invisible: 



HC1 + AgN0 3 -> AgCl | + HN0 3 . 



Uses of Hydrochloric Acid. This substance is used as a 

 source of chlorine. It is employed for cleaning_nieials. Al- 

 though present in very small proportions (about 1 part in 500) 

 in the jgastric juice of the stomach, it is a most important^ compo- 

 It is sometimes given as a medicine, when the 



natural supply is too small. 



