CHEMICAL EQUILIBRIUM 231 



The foregoing example of a reversible action, and the following 

 examples which very closely resemble it, should now be looked up 

 and studied attentively. The discussion in this and the following 

 sections, for which they furnish the basis, cannot otherwise be 

 understood: (1) the behavior of water vapor at the boiling-point 

 (pp. 63-4) and at 2000 (p. 67) ; (2) the depression of the vapor 

 pressure of a liquid by a non-volatile solute (p. 117) ; (3) the action 

 of dilute sulphuric acid on common salt (pp. 127-8) ; (4) the inter- 

 action of chlorine and water (pp. 145, 224) ; (5) the ionization of 

 electrolytes (p. 182). 



Explanation in Terms of Molecules. Restating these 

 reactions in terms of the molecules will enable us to reason more 

 clearly about this variety of chemical change. Suppose we start 

 with the materials represented on one side only of such an equation, 

 say the hydrogen chloride and oxygen in that on p. 230. The 

 molecules of these materials will encounter one another frequently 

 in the course of their movements. In a certain proportion of these 

 collisions the chemical change will take place. In the earliest 

 stages there will be few of the new kind of molecules (say, of 

 chlorine and steam), but, as the action goes on, these will increase 

 in number. There will be two consequences of this. In the first 

 place, the parent materials (in this case, hydrogen chloride and 

 oxygen) will diminish in amount, the collisions between their 

 molecules will become fewer, and the speed of the forward action 

 will therefore become less and less. In the second place, the in- 

 crease in the number of molecules of the products will result in 

 more frequent collisions between them, in more frequent occur- 

 rence of the chemical change which they can undergo, and thus in 

 an increase in the speed of the reverse action. The forward action 

 begins at its maximum and decreases in speed progressively; the 

 reverse action begins at zero and increases in speed. Finally the 

 two speeds must become equal, and at that point perceptible change 

 in the condition of the whole must cease. 



