232 SMITH'S INTERMEDIATE CHEMISTRY 



The most immediate inference from this mode of viewing the 

 matter is, that the apparent halt in the progress of the action 

 does not indicate any cessation of either chemical change. Both 

 changes must go on, in consequence of the continued encounters 

 of the proper molecules. But since the two changes proceed 

 with equal speeds they produce no alteration in the mass as a 

 whole. In fact, the final state is one of equilibrium, and not 

 of rest, one of balanced activity and not of repose. Hence, 

 chemical changes which are reversible lead to that condition 

 of seemingly suspended action which we speak of as chemical 

 equilibrium. 



Chemical Equilibrium and its Characteristics. The de- 

 tailed discussion of the relations of liquid and vapor (pp. 62-64), 

 and of saturated solution and undissolved solid (pp. 120-121), 

 has already familiarized us with the term equilibrium and its 

 significance. We can, in fact, apply to the discussion of any kind 

 of reversible phenomena, the sets of ideas in regard to exchanges of 

 molecules there elaborated. 



In particular, the reader will note that the three characteristics 

 of a state of equilibrium, developed and illustrated in the case of 

 the physical equilibrium between a liquid and its vapor (p. 63), 

 apply also to a typical case of chemical equilibrium, such as that in 

 Deacon's process now before us. Thus: 



1. There are the two opposing tendencies, which ultimately 

 balance one another. Here they are the tendency of the steam 

 and chlorine to produce hydrogen chloride and oxygen, and the 

 tendency of the hydrogen chloride and oxygen to reproduce steam 

 and chlorine by this interaction. 



2. At equilibrium the two opposing tendencies or activities are 

 still in full operation, although their effects then neutralize one 

 another. 



3. (and this is the chief mark of chemical, as it is of physical 

 equilibrium). The system is in a sensitive state, so that a change 



