116 PRINCIPLES OF CHEMISTRY. 



On the other hand, when hydrochloric acid gas is passed into a saturated 

 aqueous solution of sodium sulphate until no more is absorbed, nearly all of 

 the sodium is precipitated as sodium chloride, because the latter is almost 

 insoluble in concentrated hydrochloric acid solution, and sulphuric acid is 

 liberated and remains in the solution. In this case one of the factors in the 

 above equilibrium equation is practically removed from the field of action by 

 precipitation, thus allowing the reverse action to proceed nearly to completion, 

 and leaving the impression that hydrochloric acid is a "stronger" acid than 

 sulphuric. 



Mass action. The vigor and extent of a chemical action depends upon 

 the freedom with which the molecules can clash as well as upon the affinity 

 between substances. Hence it is found that chemical action is aided far better 

 in homogeneous mixtures, as when the substances are present in the gaseous 

 state or in solution. Such physical systems as gas arid solid, gas and liquid, 

 liquid and solid, solid and solid, offer only limited contact between molecules, 

 and, therefore, more or less impede chemical change. In homogeneous mix- 

 tures, in the case of reversible actions, the proportion of the substances changed 

 chemically is different in different cases. The range extends all the way from 

 slight change to nearly complete change. But in each individual case the 

 amount of transformation is found to depend upon the concentration of each 

 substance as well as upon the affinity between the substances. This is often 

 called the Law of Mass Action, which may be stated thus : The amount of a 

 chemical change taking place in a given lime will be dependent upon the molecular 

 concentration of each substance. 



In chemical operations it is usually desirable to obtain one or other of the 

 products of a chemical change in as large a yield as possible. If the action 

 employed is a non-reversible one, little difficulty will be experienced in obtain- 

 ing a full yield. In reversible actions, according to the law of mass action, 

 the amount of the new product formed can be increased in two ways, either 

 (1) by increasing the concentration of one or the other of the reacting sub- 

 stances, or (2) by removing one or the other of the products formed. The 

 second method namely, the removal of one of the products of the action, thus 

 affecting the equilibrium of the system in such a way that the action tends 

 toward completion is the more effective way of increasing the yield. This is 

 most conveniently done by selecting such actions that automatically remove 

 one of the products of the system in the form of an escaping gas or an insol- 

 uble body (precipitate 1 ). 



As instances of the removal of one of the products, and, therefore, more or 

 less complete action, may be mentioned the formation of all the hundreds of 

 insoluble metallic salts which are produced by the action of one salt solution 

 upon another salt solution, the first solution containing a metal which, with the 

 acid of the second solution, may form an insoluble compound, which is then 

 invariably produced as a precipitate. For instance: Calcium carbonate, 

 CaCO 3 , is insoluble ; if we bring together two solutions containing a soluble 

 calcium salt and a soluble carbonate, such as calcium chloride, CaCl 2 , and 

 sodium carbonate, Na 2 CO 3 , calcium carbonate is precipitated. 



1 The term precipitate is used to designate an insoluble substance which separates by chem- 

 ical action in a liquid, while sediment is applied to the collection of insoluble matter that may 

 be floating in a liquid, and does not imply chemical action. 



